Return to post-DH version of essay
But Snape is just nasty, right? (pre-DH version)
a consideration of the evidence for Our Hero being exceptionally unpleasant
[with thanks to Wynnleaf, duj, Dyce, Verity Brown, Red Hen, excessivelyperky, cinnamon_luvr and Mary Johnson for their input]
N.B.: This essay was quite "tight" when I wrote it, but as it got more thoughts added to it so it became more prolix. I have therefore re-edited it, tidied it up and condensed it back to readable length. Page references refer to the UK hardback editions.
The information that young!Snape was fascinated by the Dark Arts comes only from Sirius. Sirius does seem reasonably honest, and he does say it twice (once in the cave scene in GoF, once when he and Remus are reassuring Harry about the bullying which he saw in the Pensieve), which gives it slightly more weight. But Sirius must know Dumbledore well, since they were in the Order of the Phoenix together, yet in the same cave scene he says Dumbledore would never have hired Snape if Snape had ever worked for Voldemort. To the best of our knowledge Snape was a real Death Eater, at least for a few months, and Dumbledore did hire him, so we know Sirius isn't always accurate.
Sirius can't speak about Snape without being gratuitously insulting ("Slimy, oily, greasy-haired kid"; "little oddball" etc.). True, Remus does not contradict Sirius when he says that Snape was "up to his eyes" in the Dark Arts, but we've seen that Remus will suppress vital facts in order not to cause a scene, and that he was prepared to go along with Sirius's boyhood bullying of Snape. Once Sirius is no longer there to sway him, Remus tells Harry that both James and Sirius had "an old prejudice" against Snape. JK Rowling herself has said on her website that "Sirius claims that nobody is wholly good or wholly evil, and yet the way he acts towards Snape suggests that he cannot conceive of any latent good qualities there."
Sirius says that "Snape knew more curses when he arrived at school than half the kids in seventh year". This is a deeply traumatized man in his mid thirties, trying to remember an impression gained when he was eleven about somebody he is deeply prejudiced against. Nevertheless, let's assume it's true and see what it tells us.
The corollary is that Snape knew fewer curses than half the seventh years, or the same number. So Snape is being accused of precocity rather than monstrousness. And we know he was precocious, because he invented Levicorpus in the margin of a sixth-form text-book, and yet Remus tells us that Levicorpus enjoyed a vogue at Hogwarts for some months during their fifth year. Ergo, Snape was using a sixth year text as a note-book quite early in fifth year.
[We can assume Advanced Potion-Making was a NEWT text when Snape was at school, because the fifth-year Potions class he later teaches is considered quite advanced, yet when they start Advanced Potion-Making in sixth year the recipes in it are new to them.]
Knowing a lot of curses does suggest a preference for combat spells, but several curses are either on the Hogwarts curriculum (Reductor) or readily available in the library (Petrificus totalus and Locomotor mortis). A student may know several curses and yet not know anything which the school deems unacceptable. And how many curses is more curses than are known to half the new seventh years?
At the end of sixth year, Harry - who is such a Defence Against the Dark Arts expert that he's competent to teach it - knows Reductor, Locomotor mortis and Petrificus totalus. He knows of the three Unforgivables but can't cast them effectively. And he knows Sectumsempra, which may or may not be considered a curse.
So that's three definites, all sanctioned by the school, three sort-ofs and a maybe. On this evidence, "knows more curses than half the seventh years" probably means "knows six curses".
[Harry also knows Impedimenta, but only once, in the US edition of GoF, is it called the Impediment Curse, and this seems to be an error. Everywhere else - including the same place in the UK edition - it's called the Impediment Jinx.]
What contemporary evidence is there? Young!Snape writes long answers for his DADA OWL, suggesting his interest may be defensive rather than offensive. When Lily demands James tell her why he persecutes Snape, at which point coming up with a good answer would be strongly to James's advantage, James does not think to say "because he's a Dark wizard".
Of several spells we know young!Snape invented, only one is at all sinister. True, Sectumsempra wasn't actually worked out in his Potions text, so he might have kept a separate journal for Dark spells: but all his other spells that we see are humorous and similar to the Twins' tricks, except less nasty (making someone's toenails grow is probably less painful and frightening than making their tongue grow). If he really was malicious and Dark, surely a book full of his spells would include more than just one dodgy spell....
Much depends on how we interpret Sectumsempra. When Harry injured Draco, Snape - who probably didn't yet know which slicing spell Harry had used - said "Who would have thought you knew such Dark magic?" But in PoA, Snape also described the Marauder's Map as "plainly full of Dark Magic".
If Snape is always accurate about such things, then Sectumsempra is Dark but so is the Map (a surveillance device which answers as if it could think although - as Arthur said - you can't see where it keeps its brain, and which is activated by swearing a solemn oath of wrongdoing). In that case, young!Snape was indeed working Dark magic but that fact does not set him apart, because so were the Marauders.
If Snape only called the Map Dark because he's a drama-queen who always thinks the worst of Harry, the same may be true of his comment about Sectumsempra: especially as he was overwrought at the time (we're told that he "burst into the room, his face livid", i.e. ashen with shock).
We know Sectumsempra equates to a smallish knife rather than a sword or an axe, because Harry swung at Draco with all his force, and Draco was not disembowelled, beheaded, cut in half or shorn of any limbs. This fact - that Sectumsempra is a knife-spell rather than a sword-spell - raises the possibility that it was invented as a tool, for chopping potion ingredients without bruising them or for sneakily snipping shoelaces, and was only later designated as "for enemies".
If Snape designated Sectumsempra as a weapon only after the werewolf incident then he had very good reasons. Remus transforms into a beast with paws which cannot hold a wand or, presumably, do magic yet young!Snape, who knew so many curses, was thought to be in life-threatening danger from him and needed to be rescued. This implies werewolves must be immune to most magic, so it would make sense if Severus decided he needed a weapon which inflicted direct physical injury.
After the werewolf incident the staff probably watched Sirius closely, so the Pensieve/bullying scene was probably before that: and we do see Snape give James a flick with what looks like Sectumsempra. But we don't know whether he had already designated it as "for enemies", or was just lashing out in self-defence with whatever came to hand, as Harry did.
The evidence that Sectumsempra produces cursed, unhealable wounds is weak. "Sectumsempra" means something like "dissect always", and can be interpreted at least three ways:
1) "Cut permanently" (which would suggest it caused unhealing wounds).
2) "The knife which needs no sharpening" (Staysharp is a famous brand of kitchen-knife).
3) "Sever Forever" (a play on the name Severus).
That Draco needed rapid treatment to prevent scarring tells us nothing, for we see many scars in the wizarding world. These include a substantial scar on Dumbledore's knee, despite his presumably having closed the wound immediately.
Snape used a "counter-curse" to heal Draco. There are several places in the books where it sounds as if counter-curses and counter-jinxes are specific counters to specific spells, most notably:
Hermione managed to shatter it with a well-placed Jelly-Legs Jinx. Harry wobbled around the room for ten minutes afterwards before she had looked up the counter-jinx. [GoF ch. #31; p. 529]
He groped for the potion book and riffled through it in a panic, trying to find the right page; at last he located it and deciphered one cramped word underneath the spell: praying that this was the counter-jinx, Harry thought Liberacorpus! with all his might. [HBP ch. #12; p.225]
If that were so - one curse, one matching counter-curse - then the implication would be that the wounds caused by Sectumsempra must be cursed and require a special spell to heal them. However, there are also cases where counter-spells seem to be more general. The most striking of these occurs in Umbridge's DADA class:
'He says that counter-jinxes are improperly named,' said Hermione promptly. 'He says "counter-jinx" is just a name people give their jinxes when they want to make them sound more acceptable.' [OotP ch. #15; p.283]
and also when Albus is talking to Harry about self-defence in HBP:
'If there is an attack,' said Dumbledore, 'I give you permission to use any counter-jinx or -curse that might occur to you.' [HBP ch. #04; p.59]
That certainly sounds as though counter-spells are spells in their own right, which may just happen to counteract the effect of another spell. And if that's true then the wounds caused by Sectumsempra may be simple cuts, and the "counter-curse" used to fix them may be a simple healing spell.
It did take Snape three goes with the "counter-curse" to heal the wounds. That could mean they were very hard to heal, but the way it's described - first the wounds stop bleeding, then on the second pass the wounds start to knit, then on the third Draco is able to stand up - sounds very much like what you'd expect from a Muggle surgeon treating a deep wound, beginning by sealing the blood vessels, then the deep layers of tissue, then the surface layer. The way in which Snape heals Draco is directly compared with the way Dumbledore heals his own purely physical knife-wound in the Horcrux cave:
'You are very kind, Harry,' said Dumbledore, now passing the tip of his wand over the deep cut he had made in his own arm, so that it healed instantly, just as Snape had healed Malfoy's wounds. [HBP ch. #26; p. 523]
This suggests that Snape's spell likewise healed instantly, albeit in stages: and the comparison of Snape's healing of Draco's wounds with Dumbledore's healing of his own purely physical wound tends to suggest that JKR was not thinking of Draco's wounds as malign and abnormal. This fits with the fact that the cut on James's face did not seem in any way remarkable.
So, we only have the word of a biased source that young!Snape was especially involved in Dark Arts, and the contemporary evidence (Snape's own spells, and James's failure to come up with a good excuse for persecuting him) indicates that his involvement was not as deep or notorious as Sirius claimed. Sirius says that young!Snape arrived knowing more curses than half the seventh years, but even if this is true we've no reason to think that this amounts to knowing more than six curses, or to knowing any curses not sanctioned by the school.
It is possible that Sectumsempra is a seriously Dark curse which was designed as a weapon and creates persistent wounds: equally possible that it was designed for chopping herbs, creates purely physical cuts and was re-classified as a weapon only after Snape's life was threatened by someone immune to magic. Either way, we know that it behaves more like a pocket-knife than a sword, so even if it was designed as a weapon the intention was probably more defensive than offensive.
Snape's swishing robes, lowering persona and dramatic introductions to his classes all suggest that he has a streak of dark romanticism a mile wide, and may have taken an apparent early interest in the Dark Arts not because he was truly vicious but because he was a posy little proto-Goth. As the former proprietor of a small occult shop, I can personally testify that a morbid interest in curses is absolutely normal in twelve-year-old boys.
Also, Snape was an apparently dirt-poor, working-class half-blooded Slytherin at a time when Slytherin was full of future Death Eaters. A reputation as a super-cool Dark wizard and a proven ability to invent his own hexes were probably useful defences against his house-mates, as well as against the Marauders.
In any case, a precocious ability with combat spells is seen as attractive and admirable - when it's Ginny's. Neither Harry nor the Twins are seen as Dark, despite the fact that Harry has twice tried to use an Unforgivable, and the Twins were seriously planning to let off Garrotting Gas which, we are told, is undetectable, and presumably garrottes people. Ginny's comments sound as if it does so fatally. So what makes Severus's controlled and limited weapon so wicked, whilst the Twins' plan to indiscriminately endanger the lives of a school full of children as young as eleven is good-natured fun?
Note also that in HBP Dumbledore tells Harry that Snape "returned" to him when he realized the Potters were in danger, and in GoF we see a Pensieved memory of Albus telling the Wizengamot about how Snape "rejoined our side". This does suggest that Dumbledore regarded schoolboy!Snape as naturally of the light, and probably as a potential recruit for the Order.
In the bullying scene, young!Snape calls Lily "Mudblood", but we know that he wasn't in the habit of abusing Muggle-born students, because if he had been that would have been a perfect excuse for James to offer to Muggle-born Lily. But all James could come up with was "he exists".
If Severus had been hoping to impress Lily with how cool and sophisticated he was then he would have felt the way Harry felt when Cho walked in on him two seconds after Neville's pet plant showered him with Stinksap, except with a whole extra layer of humiliation and rage. And we know he tends to lash out when he's distressed.
And again, working-class, half-blooded Severus must already have been in a precarious position in Slytherin, and if he had allowed himself to be rescued by a Muggle-born Gryffindor witch his housemates would probably have punished him - and at least the Marauders couldn't get at him while he slept. Lily's intervention probably terrified him.
Given his precarious position in Slytherins and his persecution by the Marauders, Severus needed all the allies he could get. Yet the Marauders repeatedly attacked him four on one and nobody apparently came to stand with him. The Marauders probably used the Map to get him on his own: but that wasn't the case in the Pensieve scene, yet he was isolated. So his friends were either not present, not in his year, or not good friends.
Of the crowd we're told he hung around with, Bellatrix and Lucius, at least, were a lot older than him. Lucius was born between September 1953 and September 1954 (he was 41 in mid-September 1995), while Snape was born in January 1958 or 1959, so Lucius was three to five years older than Severus, and Bellatrix, according to JKR's sketch of the Black tapestry, was considerably older. One wonders whether they were befriending or exploiting Severus: it's quite possible Lucius abused him.
Sexual relationships between boys are common at British boarding schools, and even commoner in people's idea of British boarding schools. The two concepts are so linked in popular belief that JK Rowling may well be hinting at it deliberately. Sirius certainly seemed to be digging at some sore point when he called Snape Lucius's "lapdog", although if young Severus was Lucius's victim and Sirius knows it, Sirius is being exceptionally vicious even for him.
On the other hand adult Lucius probably regards Snape as a friend, and Narcissa certainly does, so if there was a sexual relationship Lucius at least doesn't see it as having been abusive. But at best there would always be a strong suspicion of exploitation, because Lucius was so much older. Snape would have had to be markedly underage if they had such a relationship at school.
The bullying in the Pensieve scene is particularly nasty. The nickname "Snivellus" seems well-established, i.e. the bullying has been going on for a long time, and it implies that the Marauders can make Severus cry, and enjoy doing so. Hanging him upside-down in public and pulling his pants off, or even threatening to do so, is a minor form of sexual assault. When he swears at them, James forces him to eat soap - effectively saying to him "You're so low, we can do anything we like to you and you aren't even allowed to protest." This is the kind of bullying which often ends in the suicide of the victim.
And to cap it all, James then tells Lily, in Severus's hearing, that Severus's crime is that he exists - letting him know that there is nothing he can ever do or say which will get the bullies off his back: except to leave Hogwarts, kill himself, kill them, or get the bleeders expelled.
As to what JKR herself thinks of the Marauders' behaviour, it's probably no coincidence that Dudley's gang includes a boy called Piers (an old form of the name Peter), who resembles a rat and who pins Dudley's victims so Dudley can hit them.
Even if the Marauders left Snape alone he probably would go on hexing them until he'd proved to his own satisfaction that he'd won. Snape says at the end of HBP that James only ever attacked him four on one: we don't know if this means that all four Marauders were still pursuing him in seventh year; or whether he's forgetting that James sometimes came at him one on one; or whether he indeed doesn't see their rivalry in seventh year as being attacks on himself by James.
However, we know that James concealed his continuing hex-war with Snape from Lily. If it had been self-defence on James's part, or even evenly-balanced, why lie to Lily about it? And the fact that he was able to conceal it from Lily strongly suggests that James picked the venue for their confrontations, using the Map, and that he was the main or even the sole instigator.
Snape, Remus and Sirius all confirm their long-standing hex-war, and yet we do not see any sign of it in the detention-notices which Harry copies in HBP, although he sees other cases of the Marauders hexing people. He never apparently comes across Snape's name, either as victim or attacker. This suggests that the Marauders used the Marauder's Map (developed some time after they became Animagi in fifth year, and lost to Filch some time before the end of seventh year), or a precursor of it, to catch Snape where there were no staff witnesses. But Snape had no such Map, so if he'd stalked them as pro-actively as they stalked him he would have been caught and presumably given detention at least sometimes. That suggests that Snape mostly just hexed them in self-defence when they had cornered him, after they had made sure the coast was clear.
Snape did indeed join a vicious terrorist organization. However, teenage boys are prone to idiot politics, and extremists routinely target lonely, troubled teenagers, offering companionship and respect. There was a lot of peer pressure on him to join, since many of his associates from school became members; he has a darkly romantic streak which might have made an outlawed, proscribed group seem appealing; and he was intensely nosy. He didn't necessarily join because he wanted to terrorize people: he may have joined "just to see", and then found that there was no way out.
We also don't know how the Death Eaters presented themselves to new recruits. They must have had some policies which sounded superficially reasonable, or they wouldn't have garnered so many members; and they probably told potential recruits that any atrocities were the work of an unrepresentative fringe element, and not authorized by charming, plausible Tom.
Also, British Wizarding society routinely subjects prisoners to life-threatening physical neglect, and to extreme psychological torture by Dementor, until the Dementors downed tools: Azkaban under the Dementors was a sort of death-camp which few survived for long. The right to a fair trial, or to a trial at all, is very fragile in Wizarding society; and at about the time Snape took the Dark Mark the Aurors were killing and torturing suspects with Ministry approval, and there was little to choose between the Head of the Department of Magical Law Enforcement and the Death Eaters.
We are not talking about a free democracy menaced by terrorist insurgents: more like the sort of South American country where the government in City Hall is nearly as brutal as the rebels in the hills. And although the Death Eaters were more racist than the Ministry as regards Muggle-borns, they were actually less racist as regards non-humans such as werewolves and giants. Even if that was just a ploy to win non-human support, it may have enabled them to present themselves to potential recruits as being the faction who were against unreasonable prejudice. Joining the Death Eaters would therefore not seem as obviously terrible a choice at that time as it does in hindsight.
How long was Snape a true Death Eater?
Several references indicate that the prophecy was made around early 1980. We know Snape was a true Death Eater at that time (unless Albus was lying). We know he defected to Dumbledore after the Potters were targeted - presumably after Harry was born, since only then could Voldemort be sure Harry would be born in late July rather than early August.
Sirius accuses Peter of passing information for a year before the Potters died, i.e. since Harry was about three months old. If the information about a leak in the Order came from Snape then he must have defected not long after Harry was born. Albus says Snape turned spy "at great personal risk", which sounds as if he'd been spying for a substantial period before Voldemort fell. We can say with some confidence, therefore, that Snape was a true Death Eater between early 1980 and August 1980, but probably no later than November 1980.
We don't know how long he'd been a Death Eater before hearing the prophecy. It could have been four years - or four days. At Spinner's End Snape says that when Voldemort was reborn, in summer 1995, he, Snape, presented him with sixteen-years'-worth of information on Dumbledore. That could mean he began spying on Albus in 1979, before the prophecy - or that he knew Albus in some innocent capacity (especially as Albus speaks of him rejoining "our side") but is presenting it to Bellatrix as early work for Voldemort.
So, we only definitely know Snape was a true Death Eater for about five months, from March to August 1980, although it is likely that he was one for a month or two after that time and at least some months before it.
We do not know whether he was a keen Death Eater until the threat to the Potters changed his mind; or whether, like Regulus, he developed doubts soon after joining, and the threat to the Potters simply moved him from "I wish I could get out of this, but I'm too scared" to "I'm going to get out of this even if it kills me."
The fact that we don't know what he did between leaving school and hearing the prophecy doesn't prove he was working for Voldemort. We don't know what anybody from that academic year did between leaving school and 1980, except that the Marauders and Lily at some point joined the Order of the Phoenix, and James and Lily got married and had a kid.
Snape is a very noticeable guy. He has long, jet-black hair of a distinctive style and texture (i.e. greasy). His eyes are unusually black. He has excessively pale skin. He has a huge and oddly-shaped nose. He has uneven, yellow teeth. He has a very striking voice. He has a dramatic and noticeable gait. Even if the Death Eater mask and hood covered him almost completely, the mask would still have to be specially shaped to accommodate his enormous hooter, and he would still be a tallish guy with a prowling walk and a very distinctive voice.
If Snape had been on a lot of raids, or standing at Voldemort's right hand torturing suspects, surely somebody would have remarked on him. And yet Sirius - who loathes Snape, and who was an Order member with access to inside information - had never heard even a hint that Snape was a Death Eater; and in the Spinner's End scene Bellatrix accuses Snape of being all talk and no action.
When JK Rowling said of Snape: "He can see Thestrals, but in my imagination most of the older people at Hogwarts would be able to see them because, obviously, as you go through life you do lose people and understand what death is. But you must not forget that Snape was a Death Eater. He will have seen things that...."
That is, she specifies that he has seen atrocities while he was a Death Eater, but she does not suggest that he committed atrocities, and takes pains to point out that the fact that he has seen death doesn't set him apart from the other staff members. It certainly doesn't sound as though she thinks of him as a willing, casual killer.
So, we know that as a Death Eater Snape did witness atrocities, but he may never have taken an active part: he certainly didn't make a habit of doing so. And we know that latterly, as a spy, he has done his best to stay out of direct physical action - which tells us that he certainly isn't, as sometimes portrayed in fanfiction, a monster of sadism who used his position as spy to get his jollies.
We have no reason to think that Snape knowingly betrayed a baby to its death. The part of the prophecy which he heard just says "The one with power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the seventh month dies." Although the champion has to be young enough for their parents to have defied Voldemort three times, that could have happened in the 1950s. There's nothing to suggest that the birth hasn't happened yet (it's only the second half of the prophecy, which Snape didn't hear, which says "will be born"), and "The one with the power ... approaches" sounds more as if it refers to an adult champion who is on the move and travelling nearer.
If anything, the fact that Snape was a Death Eater is evidence that he isn't a monster. We have seen young!Snape lash out in rage and bitterness. He was a true Death Eater for at least five months, possibly a lot longer, and he's been a fake one for years. All that time, he had every opportunity to torture and rape and kill, and his comrades probably actively encouraged him to do so. Acting the monster would have strengthened his position with the Death Eaters and with Voldemort - doubly important once he became a spy, when any loss of their confidence in him could have resulted in a miserable death by torture. And once he became a spy, he could probably have committed any crime he liked and got a free pardon for it, because it was "necessary to maintain his cover". For years he has had both opportunity, motive and encouragement to give his every base instinct free rein. And the only reputation he seems to have managed to acquire is that as a Death Eater he is all talk and no action.
Not to hurt people when you are pottering along with your safe civilian life may indicate nothing but laziness and lack of opportunity. Not to hurt people when you are burning up with rage and you have every opportunity to do so, everyone around you is encouraging you to hurt them and doing so would be to your advantage argues a profound determination not to hurt people.
Snape undoubtedly has an abrasive teaching style; and we know this is counterproductive in some cases. We are told that both Harry and Neville are more relaxed during their Potions OWL practical than they are in class, because Snape isn't there, and Harry at least does better as a result. Harry also performs better in class when Snape decides to ignore him after the Pensieve incident.
There seems to be no teacher-training in the wizarding world, so Snape is making it up as he goes along. Overall, his methods do seem to work. He tells Harry's year that he expects most of them to pass at OWL, and even Umbridge, who is trying to find fault, comments that his fifth-year students are quite advanced for their age.
Arguably, he has to pay especially close attention to both Neville and Harry because he knows Neville may cause a dangerous explosion by accident, and suspects Harry of having caused a dangerous explosion deliberately in second year. And being very nervous and tense he can't stop himself from picking at them as he watches them.
Snape certainly isn't the only teacher to use sarcasm. It was Flitwick who set a student the line "I am a wizard, not a baboon brandishing a stick." And neither of them is as sarcastic to their students as McGonagall is to Trelawney at the Christmas dinner in PoA.
At twenty-two, Snape was teaching sixth and seventh years who had been first and second years when he was in sixth and seventh year, and knew him as Snivellus, the bullies' favourite victim. It must have been nightmarish, and it's perhaps not surprising that he got into a bad habit of ruling by fear.
[Even if you take the 1958 birthdate for Snape, he started teaching at twenty-three, and the seventh years would still have been firsties when he was a seventh year.]
It's not clear whether Snape is abrasive and overbearing in class because he is a bully, or just because he still feels surrounded by enemies, and lashes out. Certainly any Potions class does need to be monitored closely, since the children are dealing with dangerous substances and an accident could kill - and not just the person closest to the cauldron. And teaching is hideously stressful - like standing up on stage to sing and having the audience continue to talk amongst themselves. A friend of mine quit teaching after she actually picked an annoying pupil bodily off the floor by his hair. At that point she decided she had to change careers, because if she stayed any longer she might actually murder one of the little brutes.
Part of Snape's problem is that he doesn't seem to know whether he's a teacher or a student. One minute he acts like a kid - calling Hermione a know-it-all, which we're told the children all do - but if the kids respond in kind he gets touchy and pulls rank.
The messy detentions he sometimes sets - e.g. cleaning bedpans without magic - also seem rather childish. But considering his comment about "foolish wand-waving", there may be a serious intent to teach the value of work done with the hands.
That we know of, he is the youngest permanent faculty member by far, living and working where he's lived since he was eleven, with colleagues many of whom taught him when he was eleven, and who probably gave him detention not long ago. Dumbledore, at least, seems to veer between treating him as a colleague and treating him as a child, so it's natural that he himself sometimes acts a bit confused about it.
His insistence on Harry calling him "sir" seems stuffy, but he doesn't object, in HBP, when Lavender or Hermione don't call him "sir". During the Occlumency lessons he gradually stops insisting on Harry calling him "sir", when Harry is talking to him as an Order member and being fairly friendly.
It seems likely, therefore, that he insists on Harry calling him "sir" as a mark of respect because he knows that when Harry omits to do so, it's usually because Harry doesn't respect him. Similarly, Dumbledore insists on young Tom calling him "sir" or "Professor", although he is much less stuffy with Harry and co..
Yet, Snape is remarkably lenient over that "You don't have to call me 'sir'" stunt. If someone tried that on McGonagall they'd get a lot worse than one detention. Either Snape is really quite soft, or he's being lenient because of Harry's recent bereavement, or he's privately thinking "Nice line - wish I'd thought of it".
As a teacher, he occasionally makes mistakes and can be inconsistent - taking points off Harry and Ron in their first lesson for not helping Neville with his potion, then later taking them from Hermione for helping him. [It's true that in the first lesson they were working in teams, and later they were working solo: but Neville wasn't on Harry and Ron's team.] He was wrong in PoA to say that Kappas mainly live in Mongolia (they're Japanese), and he causes confusion in HBP when he asks Harry how you tell the difference between an Inferius and a ghost, and then gets angry when Harry very reasonably replies that one is transparent and the other isn't. It seems clear that Snape meant "What is the difference between...?" rather than "How do you tell the difference between...?" and hasn't realized that he misspoke.
But these inconsistencies may be due to being fuzzy-tired, rather than capricious. Snape has a more gruelling schedule than any other teacher except McGonagall. I won't go into the calculations here, but I make it at least a seventy-hour week, comprising twenty-seven hours in class, six hours setting up lessons in advance, about twenty hours marking essays plus administrative duties for the subject - working out timetables, stocktaking etc. - plus both pastoral and administrative duties as Head of House, plus Order duties, plus in Snape's case whatever he pretends to be doing for Voldemort, and in McGonagall's case her duties as Deputy Headmistress. On top of which, Snape also patrols the school.
Of course, they living in and don't have to waste hours commuting, but even so it's a killer schedule, and Snape's involves actual physical danger, and hence extreme stress. It's not surprizing if he occasionally misspeaks or tells somebody to do the opposite of what he told them last week.
Nevertheless, although Snape is tired and scratchy he takes his duties seriously. He speaks of his subjects, both Potions and DADA, with passion. He tells the students where they go wrong, amongst the sniping, and he has an encyclopaedic knowledge of potions and seems always to be able to spot what mistake they've made to cause every possible change of colour or consistency. He comments on essays, showing he has read them properly, and he seems to mark reasonably fairly whether or not he likes the student. It seems clear from the hints she drops that he marked Hermione "Exceeds Expectations" in the mock-test prior to OWLs, and after the Pensieve incident, during the lesson in which Harry's potion-sample gets broken (of which more anon), Harry expects that Snape will give him an E for a good potion. Even though he knows Snape is furiously angry, he doesn't expect that prejudice will affect his marking.
Snape doesn't explain Occlumency very well to Harry - but putting psychic phenomena into words is like trying to describe yellow, especially if it just comes naturally to him. This may well be the first time Snape has had to explain, or teach, Occlumency to another person.
And despite his killer schedule, he finds time to patrol. He probably does get spiteful pleasure out of catching people out, but in a vast, rambling building where there are so many dangers (not just basilisks and Cerberuses but trick steps you could snap your leg in), and so many bullies, making sure that everybody gets back to their dorms safely is sensible and responsible.
Snape's harsh criticism of Neville is counterproductive, but it is all relevant to his performance in class: he never attacks him for his brain-damaged parents or his general vagueness. He perhaps thinks Neville will work harder to prove him wrong out of spite, because that's what he'd do.
He presumably doesn't know that Neville's family used to put him in actual danger of death in an attempt to squeeze more magic out of him. This is probably why Snape is Neville's Boggart - not because Snape is so dreadful that he is the main fear-trigger to a boy whose parents were tortured into insanity, but because in Neville's experience if you're useless at magic people try to kill you, and your family would rather see you dead than a Squib - so Snape's criticism represents threat-of-death and rejection-by-family.
There seems to be no teacher-training in the wizarding world, and no apparent awareness of learning difficulties. Snape probably teaches as he was taught. His behaviour to Neville is not really worse than McGonagall's, when she severely punished a boy with known memory problems for having a bad memory. He wasn't even careless with his password-list: he could hardly expect Crookshanks to burgle the dorm. Yet she gave him detention, banned him from Hogsmeade, and made him wait in the corridor, with a supposed murderer possibly loose in the school, until a classmate let him into the common room - which we're told was specifically intended as punishment, not a security measure. It's small wonder if Neville still believes that if he's bad at magic he'll be put in danger of his life.
McGonagall admits she was also hard on Pettigrew. Snape probably learned his teaching style from her, except that he's rattier and not so smooth: being over thirty years younger and less experienced, and much more stressed-out. But at least Snape never made Neville humble himself in front of his classmates, nor does he punish him for his vagueness - just nags him. He also protects him - from blowing himself up, and later from being throttled by Crabbe.
Snape may have reason to be tense around Neville: we know he was investigated by the Wizengamot at a time when the Aurors were authorized to torture suspects, and that Neville's parents were Aurors at that time. And he presumably knows that either Harry or Neville is the boy prophesied to bring Voldemort down, and that their failures make Tom's victory and his own death more likely.
Only twice does Snape's behaviour towards Neville seem really extreme. Once is when he tests a potion on Trevor. But bringing a poorly-controlled pet into a class full of fires and dangerous chemicals is very inappropriate, and callousness towards non-human animals seems to be the norm at Hogwarts (e.g. turning hedgehogs into pincushions and then sticking pins in them when some of them are still clearly aware and in pain).
We also see, from the Swelling Solution incident in CoS, that Snape keeps antidotes handy for whatever is being brewed, so if Trevor had been poisoned he could probably have reversed it, and there may well have been nothing lethal in the ingredients.
It was certainly unkind to criticize Neville to Lupin in such a sneering way in front of the class, but the advice is sound: Neville is a poor performer who needs close supervision in a subject which may be actively dangerous. Snape may even be trying to put Lupin off from involving Neville in the Boggart-session because he is afraid that Neville's Boggart will be his parents screaming under torture, which he can't say openly to Lupin in front of the class and has no opportunity to say to him in private.
There are also reasons why Snape may have been in an unusually foul mood. We know that he was nearly mauled by Lupin when he was a schoolboy, and that he witnessed Lupin's transformation before James intervened. We then see, in PoA, that Snape is still very scared of Lupin. He watches him neurotically at dinner; he urges him to take extra Wolfsbane (just to make sure he won't have to see that transformation again); and when he leaves after giving him the Wolfsbane he backs out of the room without taking his eyes off him. Lupin may well be Snape's Boggart.
And there is Snape, sitting alone in the staff room with an odd look on his face (glittering eyes and sneer seemingly already in place) and a Boggart in the cupboard, probably thinking about Lupin and whether he can face the Boggart, and then Lupin walks in to conduct a Boggart-facing session and Snape doesn't want to witness it. It's perhaps not surprising if he's very tense - and being Snape, when he's tense he lashes out at the nearest target.
Lupin is perhaps the better teacher: but in time of war, which would you prefer - the sour, overbearing master who will risk his life to save you, or the sweet-tempered, patient one who will risk your life to save himself? And Lupin's own life wasn't even in danger. He believed that Sirius was a mass-murderer who was out to kill Harry, yet he kept the information about the secret tunnels and about Sirius being an Animagus secret apparently just to save face, or in pursuit of some obscure private agenda.
Snape, on the other hand, is always risking himself for the children. When Harry was a baby, Snape risked being killed by Voldemort, in an attempt to save the Potters - despite presumably knowing what had happened to Regulus. He then continued to subject himself to extreme danger, in order to spy for the Order and atone for having joined the Death Eaters.
In Harry's first year Snape headed off, alone, to protect the Philosopher's Stone while everybody else was chasing the troll. He let his colleagues think he had insisted on refereeing a Quidditch match in order to cheat, and made himself very unpopular with them - submitting himself to suffering in the staff room the same sort of isolation and scorn which Harry suffered over his entry into the Triwizard Tournament - in order to protect Harry, whom he doesn't even like.
In third year, he went alone down a long tunnel in the dark, to the place where he had nearly been murdered as a boy, to face the two people who had nearly murdered him - one of whom he sincerely believed to be both a mass-murderer and a high-ranking Death Eater, and the other of whom he knew to be a werewolf on the point of transformation, and of whom he was terrified.
Why not call a Dementor to help? Why not wait by the tunnel-mouth for Sirius and Remus to come out? He knew they'd have to come back the way they went in: he'd been down that tunnel before, so he knew it had no side-exits and it's common knowledge that the Shack itself is sealed.
The only thing which required immediate intervention, and which made it unacceptable to bring a Dementor, was the presence of the children. We know Snape sprinted the whole way, because when he saw Lupin on the Map he was almost at the edge of the school grounds and running flat-out, and yet Snape got to the Shack only about five minutes after Lupin did: even though Lupin knew the way far better than he did, and the tunnel was so low in places that a short thirteen-year-old had to bend almost double.
It's true he didn't see the children on the Map, as they had already passed the boundary, but he expected Lupin was going to meet Black, he believed Black was after Harry - his first thought would surely have been to check the Map to make sure Harry was where he should be, and find out that he wasn't. As the Order's spy he probably knew James had an Invisibility Cloak, Albus had probably told him he'd passed it on to Harry, and he certainly knew that Harry had been making mysterious partial appearances in Hogsmeade - so when he found signs of a scuffle and a dropped Invisibility Cloak at the foot of the Willow he would suspect that Harry, at least, was down that tunnel with Black and the werewolf. And he went in alone to save him, into what he thought was the most extreme danger, into a situation which must have been one of his worst nightmares made real.
In fourth year, he sprinted through the castle in his nightshirt - ignoring a break-in to his own office on the way - because he heard somebody screaming. We know the opened Triwizard Egg sounds like somebody being tortured; we know Snape's office is well down the dungeon corridor, quite far from the steps up to the Entrance Hall, and his quarters are further down because he passed his office en route; we know Harry and Filch were on the first or second floor and we know Snape arrived only two or three minutes after Harry dropped the egg. And he didn't seem to have spent any time assessing the situation to find out why somebody was screaming - it could have been a Death Eater raid or another monster, but he just charged in, because he thought somebody was hurt.
In fifth year, he taught Harry Occlumency, although doing so must have put him in a knife-edge position vis-à-vis Voldemort. Already on probation, he risked angering Umbridge further by telling Crabbe to stop throttling Neville (not a light matter - if he'd lost his job, and Albus hadn't been reinstated and re-hired him, he would have lost most of his usefulness to Voldemort).
In sixth year, he shot through a closed bathroom door, livid in the face (that is, ashen with fright), because a girl's voice had just screamed "Murder!" - probably not knowing who had been attacked or by what or whether it might kill him too. Even if he was hiding nearby to watch Draco, he would certainly know that Harry had gone into that bathroom as well. So if he shot through that door expecting to protect any particular named person, as opposed to just "some student in trouble", it was probably Harry - again.
A friend who was a secondary school teacher in the 1960s said that he thought Snape's classroom sarcasm was excessive rather than normal, but that he had seen teachers who did act like that and he considered it to be a sign of insecurity. The real-life teacher on whom Snape seems to have been partly based probably had to develop a strict, snappy classroom persona to compensate for the fact that even in his middle thirties he still looked about eighteen.
It may be instructive to look at some real-life sarcy schoolmasters. The first quote sets a context for Snape by showing the behaviour of a genuine boarding-school tyrant, albeit in the 1920s. In his essay Noulded into a Shake, the late great essayist and raconteur Patrick Campbell, 3rdBaron Glenavy, described what happened when a master known as Bill the Bull caught him reading a book on ventriloquism in class. Young Patrick is practising his smile, as the book recommends, when Bill notices.
Bill rolled slowly down the passageway between the desks. He was an enormous Welshman with a bullet head, and very greasy, straight black hair. He took a subtle and delicate pleasure in driving the more impressionable amongst us half mad with fear at least five days a week.
'Such pretty teeth,' said Bill. 'How nice of you to smile at me. I have always wanted to win your admiration.'
Bill reached my desk. 'But who knows,' he said, 'perhaps you love me too. Perchance you've been sitting there all morning just dreaming of a little home - just you and I. And later, perhaps, some little ones ...?'
He read aloud: 'To imitate a Fly. Close the lips tight at one corner. Fill that cheek full of wind and force it to escape through the aperture. Make the sound suddenly loud, and then softer, which will make it appear as though the insect were flying in different parts of the room. The illusion may be helped out by the performer chasing the imaginary fly, and flapping at it with his handkerchief.'
'Strewth,' said Bill. He looked round the class. 'We'd better get ourselves a little bit of this. Here am I taking up your time with the monotonies of constitutional history, while in this very room we have a trained performer who can imitate a fly.'
Suddenly, he caught me by the back of the neck, 'Come,' he said, ' my little love, and let us hear this astounding impression.'
Bill then drags Patrick up in front of the class, encourages the other boys to jeer at him, and forces him to try to pronounce a difficult sentence without moving his lips or facial muscles, "encouraging" him by hitting him over the head with a ruler, hard, every time he makes a mistake. This ordeal continues for a further twenty-five minutes, with the rest of the class whooping and baying every time Patrick is struck.
Snape does read aloud a very short but embarrassing article which he catches Harry reading in class, but the ordeal is over in a minute or two, and he is never violent (except when he hauls Harry roughly away from the Pensieve). He seems more like the teacher described by Wynnleaf on the Loose Canon discussion group on Yahoo.
...our local schools have a 5th grade teacher (that would be with 10 year old students), who is a man in his early thirties. He is *very* Snape-like in his demeanor in class. One of my kids -- now 16 -- characterized him as 'ruthless.' Nevertheless, most of the students really like him. He makes over-the-top threats which some of the more gullible students [cut] actually believe and they *still* like him. Parents try to get their kids into his classes because they often feel that by age 10 their children really need the disciplined study habits that he'll make sure they get. Point is, in real life very few people actually think (at least around here) that this teacher is mean, nasty, etc.
The third is from the newspaper columnist Allison Pearson, writing in The Daily Mail on 28th March 2007:
Psycho, H-Bomb, Smithy, Railey, Greaser, Doughy, Vodd. No, not a rapper remix of the Seven Dwarves, but the nicknames of real teachers who once taught male acquaintances of mine.
Powerful, sarcastic, funny, eccentric and occasionally sadistic schoolmasters, they are remembered with affection and gratitude decades later.
Indeed, one of the oddest things about Snape is that he is presented as so dislikeable, when in real life such teachers are usually quite popular. But we don't really know much about how Snape is regarded by his students at large. Hagrid does say in PS that "Snape liked hardly any of the students", and Percy says "He teaches Potions, but he doesn't want to", so he does have the reputation of being sour and unhappy in his work, but that doesn't necessarily tell us whether or not the students like him.
We know that Harry, Ron and Bill Weasley dislike him, Neville is scared of him, and Tonks is annoyed by his comments about her new Patronus. In their first Potions lesson Ron says that he has heard that Snape "can turn very nasty"; but since Ron has had no direct experience of Snape yet, this information presumably comes from his brothers - and we already know that Bill dislikes Snape.
On the other hand, although Hermione dislikes Snape's bias against Harry she does not otherwise seem to object to him, and in fact defends him to Harry and Ron. And his Slytherins seem to like him, which suggests he is a reasonably pleasant house-master.
Snape is undoubtedly hostile and sometimes unfair to Harry: the question is why, apart from the obvious "Harry looks like his dad, who bullied Snape ragged." As SGCbearcub pointed out, mutual hostility makes it less likely Voldemort will expect Snape to influence or kidnap the boy, but he does also seem genuinely to dislike him.
When Harry arrives at Hogwarts, Snape probably knows nothing about him except that he is his parents' son, and that he precipitated Voldemort's downfall. Unless Dumbledore briefed him on what Hagrid discovered, he doesn't know the Dursleys neglected Harry, nor that they raised him in ignorance of the Muggle world: and since Dumbledore hadn't briefed Hagrid on what Mrs Figg knew, he seems to be keeping information about Harry's upbringing to himself. McGonagall objected to Harry being placed with the Dursleys partly because Dudley was obviously spoilt: if she's said so to Snape, he'll be primed to see Harry's cheeky, offhand manner as due to being arrogant and spoilt like Dudley, rather than sullen and neglected.
At the Sorting Feast, Snape is probably already very uneasy about meeting the son of the man who was his tormentor, his saviour and his inadvertent victim. He's talking to Quirrel when he sees Harry glaring at him in apparent hatred. We know that proximity to Quirrelmort has made Harry's scar hurt: but just as Harry connects the pain with Snape, not Quirrel, so Snape must think that glare is for him and that Harry has been raised to hate him. Both of them probably feel uncomfortable being near Quirrelmort, but they associate that unease with each other. They give each other the creeps.
[If you go to the Artnatomy site and select "APPLICATION", "NATURALISTIC MODEL" and "LEVEL II", and then click on the facial expressions for pain and anger, you will see that they are very similar. Anger has a more open eye - but it is difficult to see the eyes of someone wearing glasses and some distance away.]
Then, Draco is a whiner, and the son of Snape's friends. By the time they get to the first Potions lesson, Draco has probably told Uncle Severus that the famous Boy Who Lived was nasty to him on the train - and, being Draco, he won't have told him what he did to provoke Harry.
So, Snape comes to class expecting to dislike Harry, and assuming Harry pre-dislikes him. He asks him questions which Harry cannot answer. Was it reasonable to expect Harry to be able to answer, or did he ask Harry impossibly difficult questions with the intention of showing him up?
We know that students get their Hogwarts letter in late July, rather than a week before their eleventh birthday, because Tom Riddle was born at New Year and when he gets his visit from Dumbledore there is no mention of Christmas celebrations, and the next year's book-list is already out. Therefore, although Hermione is nearly a year older than Harry she too has only known about Hogwarts for less than six weeks. She hasn't had much chance to load up on extra books yet, and although she probably did get a few non-course books along with her set texts, if the question which Snape asked was a really obscure one it would be a remarkable coincidence if her very first book-buying expedition had just happened to include a randomly-selected Potions book with that one in it.
Therefore, the fact that Hermione can answer the questions implies the answers are in the set books, and we know Harry had read his set books with great interest. The asphodel and wormwood question relates to the Draught of Living Death, which isn't taught until sixth year, so it was probably only mentioned en passant in the first-year textbook and it was rather harsh of Snape to ask Harry about it. But when Harry couldn't answer, he went on to ask him two increasingly simple questions, about the bezoar and then about the names of aconite. Since Snape couldn't know in advance that Harry wouldn't know the answers, it looks as if he was giving Harry the chance to save face by getting at least something right, rather than intentionally showing him up. So he must have been doubly irritated when Harry couldn't answer even the simplest question, and then was cheeky about it.
It was a little nasty to quiz Harry on the set book on the very first day, but not unreasonable, especially as Harry is the son of a Potions buff. Snape was perhaps hoping for great things from the son of Lily Potter, and was disappointed to find he'd got a dead weight instead of a star. And if he doesn't know that Harry was raised in total ignorance of his family, then he may think that Harry should have paid special attention to his Potions set-book, out of respect for his mother's memory, and his lack of interest in Potions is an insult to Lily.
There is another possible layer to that scene, although I'm not sure if JKR intended it. In the Victorian Language of Flowers asphodel - which is a type of lily - means "My regrets follow you to the grave", and wormwood means "absence". Wormwood is also a traditional/Biblical symbol of bitter sorrow, as in the phrase "gall and wormwood". It is possible that Snape is actually passing Harry a secret message meaning "I bitterly regret Lily's death", which for some reason he expects Harry to understand - perhaps there was a section on the Language of Flowers in one of the set-books, and in any case many people would at least recognize the juxtaposition of wormwood, bitterness, with a lily. If Snape doesn't know how little education Harry has, he may think that Harry has understood his expression of regret and rejected it, slapping him down.
Snape does start the class with an antagonistic attitude to Harry, since he calls him "Our new - celebrity" during roll-call, before Harry has had a chance to do anything annoying, and when Harry fails to answer the asphodel-and-wormwood question he says "fame clearly isn't everything". He seems to have a fixed and completely wrong idea that Harry is lazy and inattentive because he thinks he's too cool to work. Nevertheless, Harry is lazy and inattentive, and Snape already had reason to think so by the time he started quizzing him.
After Snape makes his introductory speech, Harry and Ron look at each other with raised eyebrows - a gesture which usually indicates query, sneering or dismissal. Snape is bound to notice, because he'll be wary of both - Harry in case he's like James, and Ron in case he's like the Twins. He doesn't know whether those raised eyebrows mean scorn for the subject or for him, dismissal because they know it all already, ironic interest in his dramatic speech or some private matter they shouldn't be discussing in class. Quizzing Harry is a good way to find out whether Harry is offhand because he knows the subject already, or because he has no interest in it. And what he learns is that Harry either hasn't read his textbook or, if he did, he paid it so little attention he can't even hazard a guess at the questions; and that Harry isn't embarrassed by his ignorance but self-righteous and cheeky about it.
By a few minutes into the first lesson Snape already knows that Harry is lazy, uninterested and cheeky and expects to dislike him and be disliked, and he also has reason to be wary of Hermione - who tried insistently to answer a question which was clearly aimed at Harry, not at the whole class. Nevertheless, once the class start brewing Snape is critical of nearly everybody, Slytherin and Gryffindor alike, except Draco. That seems to be because Draco is a genuinely skilful brewer, not just because Snape likes him.
And from there on in, it's downhill all the way. But this is perhaps not surprizing.
Harry is lazy, sloppy, cheeky and, in Potions, altogether uncooperative. Any teacher would find him annoying, and Snape is a rather short-tempered, excitable person to begin with, always flushing and blenching and snarling. Every time he looks at Harry he is presumably reminded of the fear and humiliation which James made him feel, and the crushing remorse which (according to Dumbledore) he feels about the Potters' deaths, so he is automatically tense and hot and miserable at the mere sight of the boy, and so over-reacts to the very real annoyance which Harry represents.
His dislike of Harry is reinforced by the fact that he saved Harry's life, at the cost of his own social standing in the staff-room, and not long afterwards he overheard Harry and Ron saying that everyone hated him and they hoped he was ill, or had been sacked. [This scene mirrors the one in which Hermione overheard Harry and Ron disparaging her in first year, which may be a hint that the boys will eventually learn that, like Hermione, Snape has hidden charms.]
He may also blame Harry for Lily's death or, like Remus, be angry with Harry for taking stupid risks with the life his parents died to save. A sort of sibling rivalry is also possible: Hagrid, Snape and Harry are all on some level Albus's foster-sons, and Snape may fear that Harry will supplant him in Albus's affections.
In HBP, after Harry cut Draco, we see that Snape can do a certain amount of Legilimency without performing a formal spell. He probably senses that Harry constantly lies to him, yet doesn't know that Harry is usually protecting someone else. He will know that Harry despises him - the touch of Harry's mind must feel horrible to him. If he feels the trace of Voldemort in Harry, without knowing that the two have a psychic link, he may think Harry is both deeply creepy and another potential Dark Lord.
And Harry, of course, reacts to Snape as he would to Vernon - which is understandable, but just perpetuates the cycle of I-hate-you-because-you-hate-me. One of the recurrent riffs in these books is people finding out information too late for it to do any good - Dumbledore learning how neglectful and emotionally-abusive the Dursleys were after the event; Harry not finding Sirius's two-way mirror until it was too late to use it and so on. If somebody had actually said to Harry at the outset "Professor Snape will probably be a bit prickly around you because your father treated him the way Dudley treats you", Harry would have made a special effort to be pleasant and polite and that would have broken the cycle before it had properly begun: but he didn't find out why Snape was so wary and primed-to-be-hostile until the enmity between them was too well-established to be broken. Even so, they nearly managed to get onto a more civil footing during the Occlumency lessons, until Harry's nosiness blew it.
By Christmas 1992, the Trio had engineered a potentially lethal accident (if a drop of Swelling Solution had splashed into someone's mouth they could have choked on their own tongue) which injured several Slytherins, in order to create a diversion while they robbed Snape. He must be pretty sure it was them, because the ingredients for Polyjuice were taken, and then Hermione turned up furry.
And because the ingredients were stolen from his private store he may have had to pay for them. The ingredients they stole were hard to obtain and therefore probably very expensive, and we know the Potions master's salary isn't good (Slughorn complained about it after he'd insisted on a pay-rise, although admittedly Snape probably gets a bit extra for being Head of House) - so if he did have to pay for them it would have been a distinct hardship.
By the end of third year he knows that the Trio threw him into a wall, to his life-threatening injury (any period of unconsciousness resulting from a blow and lasting more than ten minutes is potentially fatal), and then did nothing to help him. Eventually he'll understand they had to stop him arresting Sirius, but since a bad blow to the head usually wipes the memory of what happened immediately beforehand, he probably doesn't know that they were only trying to disarm him, and that bashing his head was an accident.
He was unconscious for almost an hour, so he probably does realize, from the length of time missing out of his evening and the fact that he didn't wake up in the hospital wing, that they left him bleeding on the ground and didn't seek medical assistance for him, because what they were talking about was more interesting to them than whether he lived or died - whether or not he knows that they let Sirius brutalize him while unconscious. Disarming him was necessary and knocking him out was an accident, but their failure to render assistance to an injured man showed a criminal degree of callousness, and in real life they'd probably be charged with assault.
He knows that when he heard Sirius boasting about how he had nearly murdered him and saying that he'd deserved it, and he dared to resent it, Remus sneered at him and dismissed his anger as "a schoolboy grudge", and Harry called him "pathetic". He has every reason to think that they regard his life as of no account - and he'd be right. They do.
Pause for a moment and consider what McGonagall would do to three Slytherins who deliberately injured several classmates, robbed her of dangerous substances, threw her into a wall and then left her unconscious and bleeding for the best part of an hour. Yet, the sum total of Snape's revenge is that he becomes a little more obstructive towards Harry, and makes one catty remark about Hermione's teeth. This is a bit childish, but hardly the action of a vengeful brute - if anything, it suggests that he's as soft as butter.
When Harry performs well, Snape stops criticizing him - which suggests that it is largely Harry's laziness and lack of interest in the subject which angers him. When Harry gets between Snape and Sirius in the kitchen at Grimmauld Place (being more sensible and mature than the two adults at that point) only Sirius objects.
During Occlumency lessons Snape praises Harry when he does well: a bit grudgingly, OK, but he seems to come from the industrial north of England where "not bad" is a high compliment, so the fact that he praises Harry at all is remarkable. Even when Harry breaks through into Snape's memories, and is sure he'll be punished, Snape praises him. It's only when he sees the memory of Cedric's murder, and when he realizes that Harry isn't trying to keep Tom out of his dreams, that he becomes angry.
Snape is quite chatty during the Occlumency lessons, and gradually stops insisting on Harry calling him "sir". He's more open with Harry than any other Order member except Sirius, provided you don't mind Service with a Snarl. This openness and willingness to praise may be because he has taken out the worst of his emotions about James, and so Harry's appearance no longer freaks him out so badly.
[There is an anomaly, incidentally, between the US and UK versions of the scene were Harry repels Snape with a Stinging Hex. In the UK version Snape reacts with close interest - in the US one, with contempt.]
Snape talks up getting Harry expelled, but whenever he has something serious on him he doesn't act on it. When Harry is caught with the frozen Mrs Norris, Snape is fair about his possible innocence. He must know, when Hermione turns up furry, that it was the Trio who robbed him but he does nothing about it. When they throw him into a wall he assumes they were Confunded. When he thinks that Harry has robbed him again, he simply issues a warning. Even when Harry almost kills Draco he only gives him detention - which McGonagall says is lenient.
The only time he really tried to get Harry expelled was over the flying Ford Anglia. Ten minutes beforehand he had listened to Harry and Ron saying how much they hated him and how they hoped he'd been sacked, despite Harry knowing that Snape saved his life in first year - so Snape must have felt that if they wanted him thrown out, it was reasonable for him to want them thrown out.
The rest of the time, whenever he really could get Harry into trouble he back-pedals. Talking about expelling Harry is apparently just his way of blowing off steam.
Snape is certainly biased over house-points. He takes points off Gryffindor for very minor infractions, while ignoring serious misbehaviour by Slytherins - but he does give detentions and lines to his own house. This suggests that his unfairness over house-points is due to wanting to beat Minerva to the House Cup, rather than to hatred of Gryffindors. When it comes to Quidditch we are told that Snape and McGonagall are each as biased as the other.
Harry realised how much Professor McGonagall cared about beating Slytherin when she abstained from giving them homework in the week leading up to the match.
[cut] 'I've become accustomed to seeing the Quidditch Cup in my study, boys, and I really don't want to have to hand it over to Professor Snape, so use the extra time to practise, won't you?'
Snape was no less obviously partisan; [OotP chapter #19; p. 354]
- the implication being that if he was no less partisan he was probably also no more so.
After the Quidditch match in OotP, where Harry and the Twins thumped Draco, McGonagall and Snape each punish wrongdoers in their own house: Snape gives lines to a Slytherin team member for cheating, and McGonagall gives detention to Gryffindor team members for fighting.
Most of Snape's bias about house-points etc. does seem to be part of the ritual of his friendship (or whatever it is) with McGonagall. But we also see prejudice against Slytherins by other houses, and Snape may feel that he has to balance that bias and show his house-students that he is as much for them as the rest of the world is against them. He also needs to balance the fact that Dumbledore and McGonagall give points so freely to other houses.
Snape's bias seems to be more against Harry and for Draco than against Gryffindor and for Slytherin. He cuts Draco a lot of slack - but he has probably known him since he was in nappies, and he probably doesn't know what a bully he is. When Draco flashes the POTTER STINKS badge, he makes sure Snape's back is turned - showing that he expects Snape would not turn a blind eye if he caught him.
There's a recurring theme in the books about people who dote too much on a child or a pet and so don't recognize its misbehaviour - the Dursleys with Dudley, Hagrid with Aragog and Norbert, Snape with Draco. But if it's a failing and a blindness on Snape's part, at least it's a failing which springs from too much blind affection.
Insofar as Snape does harbour unreasoning prejudice against another house, and does resent Harry for being his father's son, there too he resembles Hagrid. Hagrid tells Harry "There's not a single witch or wizard who went bad who wasn't in Slytherin" when he knows that one of the Gryffindor Marauders became a mass-murdering Death Eater (even if he's wrong about which). He tells Harry that all Malfoys are rotten because they have "bad blood", and insults a terrified eleven-year-old child and curses him with a painful and humiliating pig's tail just because he finds the boy's father annoying.
We know that Dudley was a bully, even at that age - but Hagrid didn't know that. All Hagrid knew was that Vernon was a loud-mouth, so he visited the sins of the father on the son, with far more cruelty and even violence than Snape ever shows to Harry, and with far less grievance against Vernon than Snape has against James.
So if Hagrid gets a free pass for his really rather appalling behaviour, why shouldn't Snape be forgiven his much less severe treatment of Harry?
Snape undoubtedly bears Sirius a grudge - with good reason. As a boy, Sirius set him up to be killed or infected by a werewolf (at best, Sirius didn't care if he was killed or infected) - apparently, just because he found Snape's nosiness irritating. If a real schoolboy tried to feed a classmate to a grizzly, or to infect them with Aids, would we think it was "just a prank"?
Nor is it just a "schoolboy grudge". Almost the first thing we see adult!Sirius do, after his thirteen-year absence from the wizarding world, is to boast about the murder attempt and say that Snape deserved it. And he's not just winding Snape up, because he doesn't know invisible!Snape is listening: he means it. And even if he just means Snape had deserved a fright, adult!Snape will certainly assume that Sirius meant he had deserved to die.
Even when Sirius's life depends on Snape calming down and understanding about Peter, he can't resist sneering and saying "The joke's on you again, Severus" - reminding him of years of humiliation, and of the one particular "joke" which nearly killed him. Later, Sirius bashed an unconscious, defenceless and head-injured prisoner's head against the ceiling - although Snape may not know that.
[We only have Harry's opinion that Sirius bashed Snape's head deliberately. Sirius hadn't used a wand for nearly thirteen years and now he was using someone else's wand to manoeuvre a tall, limp body down a narrow, low-ceilinged tunnel in near darkness; and the second time that Harry notices Snape's head being banged, Sirius was distracted by an intensely emotional conversation. He may just have had difficulty steering Snape. But he didn't care enough to ask for help, even though his callousness risked triggering Second Impact Syndrome - an admittedly rare condition which can kill in fifteen seconds, far too fast to summon Madam Pomfrey in time to save Snape.]
Later Sirius is sneeringly unpleasant and accuses Snape of being out to hurt Harry when Snape is, so far as we know, trying to help him. He calls him Lucius's "lapdog", which may have been exceedingly cruel of him. Although Sirius says he isn't proud of the way the Marauders treated Snape, he still dismisses young!Snape as "just this little oddball", as if that justified it. Before Harry knew about the bullying, Sirius relished calling Snape a "slimy, oily, greasy-haired kid" - making it clear he has no real remorse over the way he treated him.
Snape thinks that Sirius's attitude towards him is unchanged - and he's right. Nor is Sirius's behaviour a reaction to Snape's hostility in the Shrieking Shack, because Sirius's statement that young!Snape had deserved the murder attempt was made before they re-met. Snape, therefore, is carrying an eighteen-year-old grudge against his unrepentant tormentor and would-be-murderer. Sirius is carrying an eighteen-year-old, literally murderous grudge against Snape for having been rather irritating.
Which of them is the more petty?
Snape's behaviour in the Shrieking Shack seems harsh, and he certainly got hold of the wrong end of the stick and then hung on to it: but Harry had been equally sure that Sirius was guilty, and equally vengeful, until about ten minutes beforehand.
So far as we know Snape sincerely believes Sirius is a mass-murdering Death Eater who betrayed the Potters to their deaths, making a mockery of Snape's self-sacrificing attempt to save them, and who has now returned to kill Harry. The fact that Sirius has broken Ron's leg reinforces this impression. He believes Remus is in league with the murderer and is sacrificing children who are under his care, for the sake of a friend. [In fact Remus did worse: he endangered the children under his care just to save face.] And Snape takes protecting the students very seriously.
The reason Snape came down that tunnel, instead of waiting at the entrance or sending a Dementor in, was probably because he thought children were in danger. He must have been absolutely terrified, going down into the dark to confront the two people who had nearly murdered him, in the place where he had nearly died, knowing that Remus could transform at any time [it's clear that there's moonlight shining into the Shack, probably through a skylight.] And it must have seemed poetically apt, to repay his life-debt by saving James's son in the same place where James had saved him, and from the very same danger.
Then he heard Sirius boasting of how he nearly killed him, and saying that he had deserved it. He heard Remus claiming that the enmity between James and himself had been because young!Snape was jealous of James, rather than because James bullied him ragged. [Remus probably couldn't tell the truth with Sirius in such an excitable frame of mind - but I doubt if Snape was up to being that objective.]
Overwrought though he was, Snape initially only proposed to arrest them ("Two more for Azkaban tonight"). Then Remus called him a fool for minding hearing Sirius gloating over the attempted murder, and dismissed his pain and rage as a mere "schoolboy grudge" - as if Sirius hadn't just renewed their enmity and given him a whole fresh adult grudge to bear. And then Sirius sneered at him and said "The joke's on you again, Severus", referring back to the "joke" in which he had tried to murder him, in the place where the murder had almost succeeded - and it was only then that Snape started ranting about having them both Kissed. And why should he have any regard for their lives, when they'd just made it so plain that they had none for his?
It was cruel to bait Sirius with the threat of the Dementors - but he'd just been forcibly reminded of all Sirius's cruelty to him, and seen how little he regretted it. And he was lashing out from a position of fear, not strength. He must have been petrified, stuck in that place with his would-be killers; and if he thought his position was precarious he was right, as he was knocked out about a minute later.
Snape wanted to have Sirius Kissed for being a mass-murdering traitor, and Remus for abetting (as he thought) the attempted murder of a child, when they were both sneering at him and he was hysterical with rage and fear. Sirius and Remus planned to kill Peter for the same crimes, in cold blood, as the man grovelled for mercy at their feet. Who is the more ruthless here?
When Snape says Harry "would have been well served if he'd killed you!" he sounds terrible - but he believes that James's refusal to think he could be wrong about Sirius led to his death, and now Harry is blindly making the same mistake. And since Harry rates Snape's life so low that he has just called him "pathetic" for minding hearing his own would-be murderer boasting about the murder attempt, why should Snape rate Harry's life any higher?
[Harry really does think Snape's life is of no account. After Snape is knocked out Harry thinks that he looks "lifeless", but seems completely unconcerned.]
When Snape binds Remus with cords and says that he will "drag the werewolf", he sounds callous. But he knows Remus is an imminent danger to everyone around him - including three children, one of whom is lame. The fact that James had had to rescue him from were-Remus suggests that werewolves are immune to magic and that binding one is a sensible precaution and, as we see when were-Remus jerks his paw out of the manacle, if you're going to restrain a werewolf you need to do it tightly. And the prospect of being stuck in the Shrieking Shack with an unbound, transformed Remus must be, quite literally, one of Snape's worst nightmares.
By "dragging" Remus Snape presumably means drawing him through the tunnel under Mobilicorpus. Binding Remus would be a sensible precaution even if he didn't think Remus was in league with a mass-murdering Death Eater - and if Remus had been being responsible he would have suggested it himself. Instead, the result of Remus's carelessness was that he transformed in the presence of three children and nearly bit them.
Calling Remus "the werewolf" sounds cold, but it probably means that the thought of what Remus may soon become is preying on Snape's mind - stuck as he is in the place where Remus once nearly ate him, and being still very scared of him. He may also wish to remind the children, and himself, how dangerous Remus is and why he is being so harsh.
When Snape refuses to listen to Hermione, he again sounds harsh and overbearing. But he probably sincerely believes that she's been Confunded, and that anything she tells him will be mere babble. He knows how glibly convincing Sirius can be, and how little his word can be trusted - because Sirius once nearly lured him to his death.
Of course, he really wants Sirius to be guilty. He wants to blame Sirius for the Potters' deaths, not himself. For almost thirteen years he has been able to say "I told you he was a murderer, and you didn't listen and now he's killed thirteen people." For thirteen years, he has been able to pride himself that the handsome, debonair Marauder turned out to be a murderous Death Eater and the scrawny Slytherin geek grew up to be an unsung hero. He doesn't want to lose all that lovely sense of validation.
Later that night, Snape is still trying to have Sirius Kissed, and behaving rather hysterically. However, he was out cold for almost an hour, which indicates a really bad blow - and irrational, literally foaming-at-the-mouth rage is a common symptom of concussion.
Snape thought that Sirius was a high-ranking Death Eater (we're told that none of the Death Eaters knew the identities of all the others). He believed he had prevented this high-ranking Death Eater from killing Harry - that he had stopped one of Voldemort's minions from doing something which clearly served Voldemort's interests. In case he might have to spy again, on Death Eaters or on a reborn Voldemort, it was in his interests to shut Sirius up.
Admittedly the Order of Merlin would have made his rôle in Sirius's capture public but with Sirius gone, he might be able to come up with a version of events which would make it less obvious that he had knowingly opposed Voldemort's interests. It would be only Remus's word against his, and the word of a werewolf probably doesn't carry much weight in such a prejudiced society.
As much as Snape hates Sirius and thinks Sirius is a threat to his cover, when he brought him in unconscious he conjured a stretcher for him; contrasting favourably with the brutal way Sirius treated him when he was unconscious. Yet Sirius has far less grievance against Snape than Snape has against somebody who not only bullied him unmercifully and tried to kill him, but whom he believes to be a mass-murderer.
And when Albus jokingly suggests that Sirius had been sprung by somebody who could be in two places at once, thus warning Snape that illegal use of Hermione's Time-Turner was involved, and twinkles at him to let him know that whatever happened had happened with his approval, Snape swallows his rage, shuts up and goes away. And it's not obedience to Albus which shuts him up, because ten minutes beforehand he had been encouraging Fudge to have Sirius Kissed even if Albus objected.
It could be the fact that he could get Dumbledore into serious trouble for illegal use of a Time-Turner that silences him - or the realization that if Albus authorized Sirius's escape Albus must believe Sirius's claim of innocence, there may therefore be something in it, and as much as he hates Sirius he still doesn't want him Kissed if he is innocent. Either way, he forgoes revenge out of consideration for someone else.
Snape's actions in first giving the class hints in how to identify a werewolf, and later actually shopping Remus, are often portrayed as mere spite. But suppose that you are a teacher at a real school. One of your colleagues is a violent schizophrenic. The Headmaster assures you that this colleague is perfectly safe so long as he takes his medication, but you discover that he has forgotten to take it, and has barely been prevented from killing three children. You also learn that even when he is in his right mind, the children's safety is not his highest priority.
Do you keep quiet and hope for the best, or do you warn the children that they are potentially in danger and need to treat this colleague with extreme caution?
Snape's refusal to observe Remus's Boggart-facing session may have been protective of Remus. It seems likely were-Remus is Snape's Boggart, so his presence might have caused the Boggart to turn into were-Remus in front of the children, risking giving Remus's secret away.
True, Snape did subsequently encourage the children to read up on werewolves; but that would only rouse suspicion of Remus, whilst making sure the children had some idea of how to protect themselves if he proved dangerous. The Boggart would be much more damning. Also, Snape's decision to teach the class about werewolves came after Remus had encouraged Neville to make a laughing-stock of Snape in front of the class; so if Snape had actually backed out of the Boggart session in order to protect Remus he would be feeling understandably bitter and not at all inclined to go on protecting him.
Sirius complains near the beginning of OotP that Snape has been making "...snide hints that he's out there risking his life while I'm sat on my backside here having a nice comfortable time ... asking me how the cleaning's going --" We don't know which of them started it, although on other occasions Sirius is hostile first. With hindsight, knowing that Sirius left that hated house only to die, Snape's baiting of him looks terrible - but Snape had no way to know that was how things would turn out.
Sirius is the more overtly aggressive of the two, making unpleasant accusations against Snape, whereas Snape initially only teased him a bit. Sirius accuses Snape of planning to use the Occlumency lessons against Harry, of being an unreformed Death Eater, of being Lucius's lapdog.... Snape initially just says that Sirius finds not doing anything useful for the Order frustrating, and likes to feel involved - which is perfectly true, and not even insulting, although he says it to annoy. Snape is certainly no nastier to Sirius than Sirius is to him: it's just that Sirius is bluntly rude while Snape is delicately catty.
He doesn't say anything actively offensive - the suggestion that Sirius might have got himself seen in dog-form as an excuse to stay in hiding - until after Sirius calls him Lucius's lapdog. Quite how cruel Sirius was being depends on what the relationship between Lucius and Severus was and what Sirius knows about it, but certainly Sirius upped the conversational stakes first, and Snape only followed suit - just as Sirius drew his wand first, and Snape defended himself.
Snape is to Sirius as Harry is to Dudley. Both take pleasure in baiting their former tormentor; both try to save him when his life is endangered; both are afterwards unfairly blamed for having endangered him, when they were in fact trying to save him. The only difference is that Harry succeeded in saving his bully, whereas Snape failed; and the worst you can say is that Snape is more than twenty years older than Harry, and really ought to be a bit more mature than him.
Snape did try to protect Sirius. Immediately after the fight at the Ministry, Dumbledore tells Harry that Snape had given Umbridge fake Veritaserum "when she was attempting to force you to tell her Siriusís whereabouts." That event occurred the day after Dumbledore went into hiding. Harry has not told Dumbledore about it, he didn't mention it to Sirius and Remus and Dumbledore hasn't had a chance to speak to Snape yet since his return. [He wasn't at Grimmauld Place when Snape called there, and Snape asked Sirius to stay behind to brief Albus - clearly he hadn't had a chance to do so himself. And after the battle there wasn't time for Dumbledore to have spoken to anybody.]
So, the information about Snape giving Umbridge the fake Veritaserum and what she wanted it for must have come from Snape, during some private conference whilst Dumbledore was in hiding. Either Umbridge told Snape what she wanted it for, or he read it in her mind. He may have known, when he gave her the fake Veritaserum, that he was specifically protecting Sirius - and if he didn't know it then he knew it afterwards, and thought it important enough to warn Albus about. And he could very easily not have done. Only Umbridge knew that he knew she'd been fishing for information about Sirius. If Snape had been as callous about Sirius's life as Sirius was about his, he could simply have failed to warn the Order that Umbridge was making a push to catch him.
We get to see exactly how vengeful and grudge-bearing Snape is. In the bullying scene, Peter is shown positively drooling over young!Snape's humiliation. Then Voldemort gives Peter over into adult!Snape's hands - and he probably doesn't care that much what Snape does to him.
Snape is no angel, and he does take his revenge. But so far as we are shown, the sum of that revenge is that he bosses Peter around and sneers at him a bit.
When Snape takes points off the Trio for taking a library book outside, you certainly get the impression he's invented that rule. But he'd just been bitten by Fluffy, and was limping noticeably, which suggests that he was in a lot of pain. And he does tend to deal with stress by giving it to other people.
When Snape referees the Gryffindor/Hufflepuff match in PS, he awards a penalty to Hufflepuff because George has hit a Bludger at him, which is fair, but later he awards another penalty to Hufflepuff for, in Ron's opinion, no reason at all. At the end of the match, Harry nearly knocks Snape off his broom. Snape lands, white-faced and tight-lipped, and then is said to spit bitterly.
In PoA Remus claims that the enmity between Severus and James was because Severus was jealous of James's talent for Quidditch. We know it was mainly because James bullied Severus ragged; but the fact that Remus chose that particular lie does suggest young!Snape was known not to be as good a flyer as James. Then, Harry sees Snape's memory of child!Snape failing to get onto a bucking broom. Yet Snape has fast reflexes, he seems extremely observant and, as an adult, he moves in a way which suggests excellent coordination - all of which ought to have made him a potential Quidditch star. This, combined with his white face and tight lips after Harry nearly knocked him off his broom, suggests that Snape possibly suffers from vertigo, and that he spat because he'd just nearly been sick with fright, and had a mouthful of stomach acid.
If, however, he really was bitter, he had something to be bitter about. As we later learn from Quirrelmort, in order to protect Harry Snape had put himself in a miserable position in the staff room, allowing the people he had to socialize with and eat with to think that he had insisted on refereeing the match in order to cheat, and making himself markedly unpopular. In return, Harry nearly knocked him off his broom accidentally, and George Weasley tried to do so deliberately.
We don't know whether he awarded a penalty to Hufflepuff for no reason because he really was simply cheating; or because he needed Quirrel to think he was refereeing in order to cheat, to keep his protection of Harry secret; or because he was so bitter about being sneered at in the staff-room when he was trying to do a good deed that he thought "Well, if you all think I'm going to cheat anyway, I might as well do the crime I'm being punished for."
If he really was spitting and not retching, that was an early clue that his origins were decidedly rough. Spitting in public is neither normal nor socially acceptable in Britain, except among teenage yobs and footballers.
The "I see no difference" crack about Hermione's teeth seems spiteful and childish, but he does have a massive grievance against her. She put Harry up to causing an explosion which inflicted similar distortions on several classmates, to create a distraction while she robbed Snape; and less than five months before the tooth incident she had thrown him into a wall and then left him to lie bleeding and unconscious for almost an hour. This remark, so far as we can tell, is his only retaliation. He then takes fifty points from Harry and Ron, where he normally takes points in fives and tens, which again suggests he may have been working off the grudge from the end of last term - or perhaps that he is in an especially vile mood because he has both Moody (as he thinks) and Karkaroff to contend with in the staff room.
It's not clear that he means he can see no difference between Hermione's overgrown teeth and her usual ones, in any case. Since Ron is making a bit of a drama out of what's happened to Hermione, Snape may mean that Draco's having cursed Hermione is no worse than Harry's having cursed Goyle. [Here too we see that Snape doesn't condone bullying, because Pansy and the other Slytherin girls point and jeer at Hermione, but they do it in silence and behind Snape's back, so he won't catch them at it.]
When Snape blasted the courting couples out of the rose-bushes after the Yule Ball in GoF, he seems like a spiteful killjoy. But Karkaroff had cornered him into a conversation which could get him killed if it was overheard by the wrong ears, and some of the students have Death Eater sympathies. Clearing the bushes of potential eavesdroppers was literally a matter of life or death.
When Harry comes to tell Dumbledore that Barty Crouch Snr has been found in the Forest, rambling and mad, Snape emerges from Dumbledore's office and then behaves obstructively, telling Harry that the Headmaster is busy.
This seems irresponsible, but Dumbledore rapidly appears, and it may be that Snape knew that he would, and was just amusing himself rather spitefully by provoking Harry. "The Headmaster is busy" may well be a euphemism for "The Headmaster is taking a leak". Or perhaps Albus really was doing something which could not be interrupted.
Harry feels that Snape enjoys frustrating him while he is panicky, and the fact that Snape smiles tends to suggest that he is right, although he could be smiling over whatever Albus is busy with. But despite the hint of malice Snape is not really being any more obstructive to Harry than McGonagall is in PS, when Harry wants to warn the Headmaster that the Stone is in danger.
However, having found a missing senior civil servant wandering deranged through the woods is a more urgent matter than some vague fear that the Stone might be stolen, and if Snape is being as obstructive as he sounds, this is one point where you could make a case for saying, maybe he's really in league with Voldemort and he doesn't want Crouch Snr to be found. But that would suggest he was in on the whole thing with Barty Jnr, when in fact Barty Jnr seems to distrust him.
If Snape knew that the Headmaster would be down shortly, he would know his snideness wouldn't delay the rescue-attempt, so he wasn't being obstructive: just petty. And initially, Harry just said Crouch was ill and wanted to see Dumbledore, which Snape may have seen as demanding arrogance on Crouch's part. We never find out how Snape would have reacted to the revelation that Crouch was mentally disturbed and wished to convey a warning, because Dumbledore arrived as soon as Harry had said it; if he hadn't, Snape might have gone to look for Crouch himself.
However, Snape probably has good reason to be reluctant to help Crouch Snr. Sirius said that towards the end of Vold War One Crouch authorized the use of Unforgivable Curses (plural, so not just Imperius) on Death Eater suspects, and became "as ruthless and cruel as many on the Dark side": which implies that those Unforgivables included Cruciatus.
Snape was a Death Eater suspect during that period. Dumbledore managed to get him off, but we don't know how long it took, and we see from the imprisonments of Hagrid and Stan Shunpike that Dumbledore's power to protect people from the Ministry is limited. It is quite possible that Snape was tortured on Crouch's orders.
Whether Snape himself was tortured or not, Crouch's policies must have added an extra distress to spying. Bad enough to hand over people who trusted him to be tried and then consigned to the Dementors; but to betray his erstwhile friends knowing that because of him they'd be tortured and imprisoned without trial, and possibly summarily executed, must have added another layer of miserable guilt to the guilt of the Potters' deaths.
Nevertheless, although Snape had reasons for his obstructive attitude that day, it contributed to the debacle at the Ministry a year later. Itís not surprizing that Harry didn't go to Snape when he "saw" Sirius being tortured, because his previous experience suggested that Snape would impede him.
Stopping the Occlumency lessons seems irresponsible, yet there are very strong reasons why Snape might feel he could not continue.
First off, Harry's hatred of Snape must have made reading his mind as unpleasant for Snape as for Harry. He was also distressed (white-faced and angry) at witnessing Cedric's murder, and shaken by having his own stressful childhood memories viewed.
Two days before that final Occlumency lesson, Dumbledore had fled the school to protect Harry. Dumbledore's presence is vital to Snape, because Snape's value to Voldemort depends on his continuing to be in a position to spy on Dumbledore.
One day before it, Snape had risked antagonizing Umbridge by supplying fake Veritaserum, to protect Harry and Harry's information about the Order; and a Slytherin disappeared. Shortly before he found Harry invading his memory that student had turned up with severe psychological and possibly physical injuries. [If Montague was found in an actual toilet, rather than a toilet-cubicle, he would have broken bones and internal damage.] Snape must have been crawling up the walls with stress.
Then he sees Harry - Harry for whose sake he is spending time on this lousy, stressful occupation, Harry who seldom bothers to practise even though he knows Voldemort may at any moment look out through his eyes, Harry for whose sake Dumbledore has had to flee the school - invading his memories. OK, he has invaded Harry's mind with every lesson, but he was doing it to protect Harry, whereas he thinks Harry is just being nosy. Snape reacts violently to Harry's invasion, just as Harry protected his memory of kissing Cho.
Snape expects that Harry will have sided with James and enjoyed his humiliation, that Harry will snigger about it with his friends, that the nickname "Snivellus" will be resurrected. He must feel that the bullying has never stopped and never will stop, that James is reaching down through Harry to continue tormenting him. He thought it had stopped, he thought he was over it, and now he's right back where he started. And if he didn't notice how far into the scene Harry had got, and the Marauders really did strip him, he may think that Harry has seen his genitals, in humiliating circumstances - so he feels sexually invaded, just as he must have done at the time.
There's also a security aspect. The other memories in the bowl might be things he didn't want Voldemort to see, through Harry - in which case the fact that Harry can't be trusted not to snoop could endanger Snape's life, and his usefulness to the Order. Indeed, the way in which Harry was suddenly gripped by a powerful urge to look in the Pensieve was rather suspicious, and it may actually have been Tom who prompted him to look. And Snape knows that his own nosiness killed Harry's parents, so he may feel especially angry to see Harry of all people taking stupid risks with information which might betray somebody to Voldemort.
As to the violence of Snape's reaction - I imagine if he'd actually intended the jar to hit Harry, it would have done.
Snape certainly was not treacherously using the lessons to make Harry more vulnerable to Voldemort. Dumbledore later tells Harry that he did not teach him Occlumency himself because "I was sure, at the time, that nothing could have been more dangerous than to open your mind even further to Voldemort while in my presence --" That is, a temporary widening of the channel between Harry and Voldemort was an expected part of learning Occlumency, and Albus thinks that it would have happened had he been teaching Harry, as much as it did with Snape.
After the fateful Occlumency lesson, there follows a Potions class during which Snape is so angry and distressed that he won't even look at Harry. Harry produces a good potion for once and takes a sample up to Snape's desk, but as he walks away he hears a crash behind him and turns to find his sample-bottle broken. Snape gives him a zero, and gloats about it.
If Snape deliberately knocked Harry's sample over that was deeply unprofessional of him, whatever the provocation. But there is no clear indication that Snape did so, or even that Harry thought he did. It may have been placed too near the edge, or knocked accidentally by Snape or by another student bringing their own sample to be marked. All we know is that Snape did take a spiteful pleasure in Harry's misfortune - at a time when he was seeing him as a hateful extension of James.
When Snape meets Harry and Tonks at the gate at the start of HBP, he is rude to Tonks and brusque with Harry, who is covered in blood. This is the only time we see him unconcerned about physical harm to a student, but he probably knows that Auror Tonks will have First Aid knowledge and have fixed any immediate problems.
Regarding Snape's hostility to Harry, Draco may already have given a version of events on the train which put the blame solely on Harry, and if Snape knows Harry spied on Draco he must fear Harry has jeopardized whatever he and Albus are planning re. the Vow. He probably knows that in June Harry and the DA left Draco, Crabbe and Goyle to suffer for hours with severe curse-injuries on the same train; and, Draco being Draco, he won't know that Draco and co. started it by ambushing Harry three on one. So he will think that Draco was the innocent party and that Harry's injuries serve him right.
His advice to Tonks, though snide, may be sincere. She is a junior Order member and recent student (NEWT Potions is compulsory for Aurors) and he may really be concerned that her new Patronus is weak and could endanger her, or that Lupin the man is weak and not to be relied on - which may be sound advice, although being Snape he probably does also get malicious pleasure out of sniping at Lupin.
His manner is rude (in contrast to the smoothie act he put on for Tonks's aunts), but he has reasons for his bad mood. He's been called away from his dinner, and from the Sorting Feast at which he should oversee his new Slytherins, and made to trail across the grounds in the dark because of Harry, who must seem like a constant aggravation. And he's been unexpectedly bounced out at by a Patronus in the form of were-Lupin, of whom he is terrified, and who reminds him of all sorts of angst-ridden memories involving Harry and James.
Depending on how he was treated when he was investigated in his early twenties, he may also have good reason to be bristlingly uncomfortable around Aurors generally.
When Snape speaks to Draco after Slughorn's Christmas party, he tries stiffly to pull rank, to get Draco to tell him what he's up to; but he does make an effort to be sympathetic about Lucius's imprisonment. Draco is quite nasty back, telling him to break his Vow - which literally means "Drop dead".
This could mean that Draco really dislikes Snape, but the writer Dyce makes a good case that Draco is letting off steam at an adult he knows is safe to shout at, like any fractious teenager screaming "I hate you, I wish you were dead!" at a parent or uncle. Either way, it's clear that Draco isn't afraid of Snape; neither as a man, a teacher nor a Death Eater. This again suggests that Death Eater!Snape doesn't have a particularly fierce reputation.
Somebody on the Bewitched Mind forum commented that "Snape is good to his friends, and his friends are Death Eaters", and that he is primarily a friend to Narcissa and Draco, rather than either Dumbledore's or Riddle's man. But his friends' best interests clearly lie with Dumbledore, who wants to protect them, rather than with Tom who wants to send Draco on a suicide mission to spite Lucius.
After Harry nearly kills Draco with Sectumsempra, Snape makes Harry copy records of student misdeeds, including some committed by James and Sirius. Harry assumes Snape's motive is to upset him. McGonagall says Snape is being lenient and Harry could have been expelled (albeit Snape would hardly want Harry expelled when his own survival, and Tom's downfall, may rest on Harry's shoulders).
From Snape's viewpoint, Harry is another Sirius, killing for kicks; made worse because the victim is somebody he's probably known since infancy. Harry's hostile and self-righteous attitude will further convince him that Harry is like Sirius, attacking without compunction and stealing his spells to do it with, and he'll feel horribly guilty that his own spell injured Draco. We know that Harry's reaction to Draco's tears was pity and his reaction to his injuries was horror and remorse, but Snape doesn't know that; and neither Harry nor Draco will tell him that Draco tried to Crucio Harry, and Harry only lashed out in self-defence.
Seeing Harry apparently going the way Sirius went, it's reasonable for Snape to show Harry what poor rôle models James and Sirius were and how they spent their time on petty nastinesses (even Remus says James hexed people for fun). Which is not to deny that he takes a malicious pleasure in baiting Harry about the summer evenings which he is missing: but at this point he must really be seeing Harry as his own tormentors reborn, and is digging at Harry as Harry digs at Dudley.
It wouldn't occur to him that Harry really liked the boy in the book. With their past history he would see it as Harry stealing his spells and ideas the way James did - intruding into his private memories again, and then using his ideas to trick Slughorn into giving him better marks than he deserves.
On the rare occasions when we Snape relating to anybody outside Harry's class, we glimpse a reasonably pleasant persona. He doesn't express emotion (except anger and bitterness) openly but his gestures are "tells": we see him clutch at the chair-back when he hears that Ginny has been taken, and whiten at Harry's memory of Cedric's death.
He sympathizes with Draco over his father's incarceration even in the midst of a flaming row, and when Draco is injured Snape not only heals him but pauses to wipe the blood from his face, and reassures him that he probably won't be scarred. He deals with Narcissa's tears awkwardly but quite kindly. He sneers at Bellatrix's suffering in Azkaban - but he may feel that as a sadistic torturer she has forfeited the right to sympathy.
He skulks uncomfortably around the edges of Slughorn's party, yet he does come - presumably out of obligation or affection for his old house-master. He even lets drunk!Sluggy put his arm round him, and doesn't recoil.
Albus speaks as if they are friends, always calling him "Severus", inviting him to sample a custard-tart with him, speaking with sorrow of his unhealing emotional wounds. The scene at the Christmas table in PoA, where Snape pushes the vulture hat at Albus and Albus swaps with him, seems intimate and affectionate.
Minerva speaks of their rivalry, how she couldn't look Snape in the eye for weeks after his team flattened hers, and she says "Professor Dumbledore" but she says "Severus Snape", and the whole thing sounds more like affectionate winding-each-other-up than bitter competition.
And Remus says "We shall never be bosom friends, perhaps; after all that happened between James and Sirius and Severus, there is too much bitterness there" - but he apparently doesn't see anything odd in the concept of Severus having bosom friends per se, nor even totally to rule it out for himself.
Some fanwriters think Snape has no sense of humour, but if you "get" sarcasm you see that Snape is joking most of the time. Sarcasm is highly regarded in Britain, and is generally neither intended nor taken as seriously as it would be in a society where sarcasm is rare.
Indeed, sarcasm is common among the Hogwarts staff - McGonagall, Dumbledore and Flitwick all use it. Consider Dumbledore's comment that "[Scrimgeour] is not very happy with me, either. We must try not to sink beneath our anguish, Harry, but battle on." When the staff confront Lockhart after Ginny has been taken, Snape, with his sarcasm, speaks for all of them. He and Dumbledore and McGonagall and Flitwick probably sit around drinking coffee and bitching about people they don't like.
Snape doesn't seem able to laugh at himself, however, at least in front of students, although there may be an element of self-sendup in all that dramatic prowling and robe-swishing, and in his teasing of McGonagall. But around the children, he is too defensive to let his guard down.
Snape is no more sarcastic than Dumbledore, no nastier to Neville than McGonagall is and probably no more prone to playing favourites than Hagrid is. He only seems extra-nasty because he doesn't like Harry, and it's through Harry's eyes that we see him. Ironically Snape and Harry have much in common, and we see, through the Prince's book, that Harry would have liked Severus if they'd met as boys.
Snape behaves to Sirius as Harry behaves to Dudley - enjoying baiting a former tormentor, but still trying to save his life. Snape plasters on a sickly smile to congratulate McGonagall for winning the House Cup in PS, and Harry does the same over Ron's Prefect's badge.
Each turns from someone else's strong emotion in a fit of masculine embarrassment, described in exactly the same way. "[Snape] looked away from the sight of [Narcissa's] tears as though they were indecent". "Harry had never seen Lupin lose control before, he felt as though he was intruding upon something private, indecent; he turned away".
They also resemble each other in wanting to blame somebody else. I suspect Snape wanted Sirius to be guilty so he could blame him for the Potters' deaths, instead of himself. And Harry obsessively blames Snape for Sirius's death, which he half knows is unfair - but it's less painful than blaming himself, and less dangerous than blaming Kreacher, whom he actually could hurt.
Snape is, in his snarly way, more consciously ethical than most on the "good" side. When he did something very wrong he had the moral, emotional and physical courage to go to Dumbledore and confess, at the risk of ending up in Azkaban or being killed by Voldemort. This contrasts markedly with Sirius, still sulkily trying to justify his attempt to murder a classmate for kicks, and Lupin, who preferred to leave Harry in danger from a supposed mass-murderer rather than admit that he had broken Dumbledore's trust as a boy.
Snape feels bound by his debt to James, even though he hated James and thinks that James's motive for saving him was self-serving. He feels, according to Dumbledore, intense remorse for the Potters' deaths, though he hadn't known whom he was betraying, and had risked his life to save them.
He appears to loathe teaching, yet does it dutifully, and doesn't skimp on his work. Altogether he has a degree of moral seriousness which is lacking from most of the other characters, and is always taking risks for other people: even if he is a bit of a drama queen who makes sure those other people know what a sacrifice he's making.
People think Snape must be out for Number One because Phineas Nigellus said that in the last resort Slytherins always save themselves - but who said Phineas was truthful and unbiased? Itís true he sounds like a portentous Voice of the Author slipped into the story: but so did Firenze when he asked Harry to tell Hagrid that his attempt to civilize Grawp wasn't working - and Firenze, as it turned out, was wrong. And portrait-Phineas isn't even a real human person; just some kind of imprint of a real person.
How do Slytherins really behave? Bellatrix and Barty Jnr (assuming he's Slytherin) sacrifice themselves limitlessly for a cause, even if it's a bad cause; Regulus gives his life when morality compels him to defy Voldemort; Andromeda (Slytherin, because all the Blacks were) sacrifices family and position for love of a Muggle-born; Narcissa dares Tom's wrath to save her son; and Draco is at least as worried about saving his parents as saving himself. Clearly, being a Slytherin is no barrier to being brave, ethical or self-sacrificing. Even Horace has definite standards. The only known Slytherins we see much of and who seem wholly self-serving are Tom and Lucius.
One of the few other characters who seems to think about ethics (along with Hermione and Albus) is Sirius, who talks about how the world is much more complicated than just "them and us". But he has, as JKR herself has commented, a complete blind spot about Snape.
A discussion in the Leaky Lounge reportedly concluded that Snape was a sexual sadist. We know that he isn't a sadist in the psychopathic sense, because if he was he wouldn't have spent however-long-it-was as a Death Eater without attracting the slightest attention. As to whether he might be a mutual-game-playing, S&M-type sadist, I discussed this with someone who knows a lot of S&Mers, and his immediate response was that Snape was absolutely not the type.
His reasons were that Snape is too self-conscious and walled-off for that sort of complex two-way emotional dance, isn't "physical" enough, and since he spends his working life in complex and terrifying rôle-playing he's not likely to want to do it in private as well: quite the reverse. He's far more likely to want a straightforward roll in the hay, or wine and candlelight and a soft bed, than something kinky with whips and chains.
The belief that Snape is Eeevil depends mainly on his killing Dumbledore against Dumbledore's wishes. Is that compatible with canon?
First off, Dumbledore tells Harry that he knows about Snape's Unbreakable Vow to protect Draco (and that it causes him no disquiet), and he tells Draco on the Astronomy Tower that he knows Draco has been trying to kill him.
If Dumbledore lied to Harry, and didn't know about Snape's Vow until Harry told him, or if he knew Snape had taken a Vow to protect Draco but he had to find out for himself that Draco was trying to kill him, that would surely make him distrust Snape. His trust in Snape seems to be absolute to the end: ergo, Snape had told him about his Vow, at least as it concerned Draco, and had told him Draco was trying to kill him.
It has been suggested that evil!Snape might tell Albus about the Vow and Draco's mission in order to make himself appear totally trustworthy, to improve his chances of being able to kill him. However, Snape already had Albus's trust, sufficient to get close to him. I do not see that any increase in that trust would significantly improve his chances of killing him, especially as forewarning him would put him on his guard.
Secondly, Snape's Vow means that if he does not protect Draco to the best of his abilities, he will die. His "abilities" presumably include his ability to perceive danger.
Warning Albus that Draco was trying to kill him might increase Snape's chances of killing Albus, but it would decrease Draco's chances of doing so, and so increase the risk that Draco would be punished or even killed by Voldemort. If evil!Snape told Albus about Draco's mission - as he apparently did - Snape should have died, because he had failed to protect Draco to the best of his abilities.
But if Snape is a virtuous man, then he will see becoming a murderer and a favoured Death Eater as Bad Things, more harmful to Draco than being Crucioed by Tom. He can tell Albus about Draco's mission and live, because by doing so he has protected Draco to the best of his abilities.
Telling Albus about the Vow might make Albus reluctant to hurt Draco at Snape's expense, but it's unlikely that Albus would have hurt him anyway. Telling Albus about the Vow definitely increases his suspicion of Draco and make it less likely that Draco will kill him, and more likely that Draco will be punished by Tom. The fact that Snape clearly did tell Albus about at least part of the Vow, and is still alive, is pretty good evidence that he is good!Snape who thinks that protecting Draco's moral health is more important than preventing him from being punished by Tom.
We don't know whether Albus knows Snape swore to carry out Draco's task if Draco failed. However, if Snape was reserving the option of himself or Draco killing Dumbledore against Dumbledore's will, why forewarn Dumbledore by telling him about the Vow at all? And Hagrid heard him trying to get out of something, to back away - if he was planning to kill Albus you'd think he'd stay close and pretend to be compliant.
Dumbledore tells Draco that he didn't let on that he knew Draco was trying to kill him, because he didn't want to get him into potentially fatal trouble with Voldemort. But if his motive was solely to protect a student, surely he should have done more to protect other students from being accidentally killed by Draco? And "I'm so inept that my intended target knew what I was up to all along, and let me keep trying out of pity because he was sure I'd fail" isn't going to play too well whenever Tom sees it. If Albus fears that will cause Tom to kill Draco, why tell Draco about it at all, and risk Tom reading it in his mind? Why not pretend amazement when Draco announced that he was the would-be assassin? And if he can offer Draco some sort of Witness-Protection Scheme now, why couldn't he have done so months ago?
A simple explanation for Dumbledore's failure to prevent Draco from endangering other students is that if he did so it would then "seem Draco will fail" to carry out his commission, and that would force Snape to either kill Albus or die, sooner than either of them intended. But that presupposes that Albus did know about the third clause of Snape's Vow.
And why give Snape a cursed post whose incumbent must depart or die within a year, unless Dumbledore had reason to think Snape would die or depart within a year anyway?
So, Dumbledore probably knew about Snape's Vow, including the third clause. He knew that if Draco seemed unlikely to succeed in killing him Snape would have to do it instead, or die. If Snape stopped Draco killing him, Snape would die - since it would then certainly "seem Draco will fail".
An hour or two beforehand Dumbledore had told Harry that any risks must be his, because he was much older and less valuable than Harry. There's a war on, and Dumbledore might someday have to send Snape on a suicide mission: but given a straight choice between his life and Snape's, does anybody believe that when he heard about Snape's Vow he replied "I'm sorry, dear boy, but it was your mistake so you'll just have to die for me"? To a colleague and friend whom he has known since he was eleven, who is well over a century younger than him and who is one of the Order's best assets?
Suppose Dumbledore didn't know about Snape's Vow, or thought that sparing his life for some reason wouldn't trigger the Vow. What were the alternatives to Snape killing him?
On the battlements (pace Rowling, not the ramparts, which are the banks of earth around the base of a fortification) that night were Dumbledore himself, disarmed and too weakened to perform wandless magic; four Death Eaters including Greyback, who can probably only be killed by silver; Draco, who probably wouldn't side with Dumbledore; Harry, invisible and in a Body-Bind; and Snape.
If Snape had tried to save Dumbledore, he would have been alone against four opponents (five if Draco sided with the Death Eaters) - and we know from his confrontations with the Marauders, and in the Shrieking Shack, that he can't reliably take down four at once. The outcome would almost certainly have been Snape, Dumbledore, Harry and possibly Draco dead and Greyback loose in a school full of children with no superior officer to control him.
Forget whether Albus would be prepared to see Snape die. Forget whether there was even any possibility of Albus not dying. Harry was frozen, invisible and helpless, close to the Death Eaters' position. Only Albus knew for sure that he was there. Only Albus knew where he was. If there were to be a fight, it was very likely that Harry would be killed in the crossfire.
A couple of hours beforehand, Albus had told Harry that if there were risks to be taken he would take them, because he was both older and less valuable than Harry. Why, in those couple of hours, would he have changed his mind to the extent that he was now prepared to risk Harry's life to save his own?
For Dumbledore not to want Snape to kill him, he would have to have been prepared to risk the almost certain deaths of Snape, Harry, Draco and an unknown number of children in exchange for a minuscule chance of saving himself. Unless Dumbledore was mad, stupid or a callous megalomaniac, he has to have wanted Snape to kill him; and Snape's doing so is a proof of his obedience to Albus, not his betrayal of him. He didn't even have the option of killing himself, since he would then just be leaving Dumbledore to the non-existent mercy of the Death Eaters and loosing Greyback on the students, out of a selfish desire to preserve his own honour.
Of course, killing Albus might still have suited some nefarious agenda on Snape's part. But the fact that Snape apparently forewarned Albus by telling him about the Vow, and that Albus felt he had to beg him to obey, combined with the conversation which Hagrid overheard, is strong evidence that Snape did not want to do it.
As usual, Snape was trying to save someone else's life at the risk of his own. Otherwise, Dumbledore wouldn't have had to argue, cajole and finally plead with Snape to get Snape to kill him - Snape obviously preferring the alternative, in which he failed to fulfil his Vow, and died for Albus.
In Harry's opinion, Dumbledore was pleading from that first soft "Severus..." spoken when he saw Snape appear in the doorway.
Dumbledore wouldn't plead for his own life, but he might plead with Snape not to revert to evil. But why? A few minutes beforehand he had trusted Snape absolutely, he insisted that Severus was the only one he wanted. Why would the mere sight of Snape suddenly make Albus start begging him not to change sides? As cinnamon_luvr said, "It's like having Harry surrounded by DE's and Ron appears in the doorway. Why would Harry's first thought, then, be 'OMG! He's a traitor! Don't kill me Ron!'?"
Albus could have been pretending to beg in order to bolster the illusion that Snape had betrayed him. More likely he was pleading with him to keep a previous agreement and kill him. But he wasn't pleading with Snape not to betray him, because when he started pleading, he had no reason to think Snape even might betray him.
If the Vow only comes into play if the Bonder knows that the oath-taker has broken their Vow, it would theoretically be possible to fake Albus's death. If they had worked out such a faked death in advance, then indeed Albus would not want Snape to kill him, and if Snape did, that would be a betrayal.
But if Snape had the option of faking the murder, there's nothing to say he didnít fake it. The situation as described leaves room for Snape to have said "Avada Kedavra" without willing it (which does no harm), whilst non-verbally using Levicorpus to get Albus down from the tower. But JKR's comments in New York seem to have ruled that out - unless Albus landed alive and then died of poison.
There is another possible angle, although only the publication of Deathly Hallows will show if it's valid. If Albus expected to die before Harry confronted Voldemort, then he may have ordered Snape to kill him by magic in order that in the final confrontation Snape would be able to use Priori Incantatum to summon Albus's shade from his wand, thus ensuring that Albus would be present for the final battle.
To those who still hate Snape for killing Albus, even if it was on Albus's orders, consider this. A few hours beforehand, on the man's own orders, Harry force-fed Albus the potion from the Eldritch Birdbath of Doom. This certainly contributed to Albus's death, by making him too weak to defend himself.
If it turns out that Snape in fact did not kill Dumbledore, but merely faked his death to convince the Death Eaters, and that Albus died some hours later from the poison which Harry made him drink, will you transfer your hatred from Snape to Harry? If not, why not?
As to why Snape started at the Headmaster in apparent hatred and revulsion - if he was truly Dumbledore's man he would have hated what he was having to do. But also, it was probably the fulfilment of the Vow which Hagrid heard him arguing about with Dumbledore, and Snape saying Albus took too much for granted and he wouldn't do it. He'd be furious with Albus for emotionally-blackmailing him into doing something he'd already said he wouldn't do. This is Snape we're talking about, after all - and nobody ever claimed he had an easy temperament.
But if the fact that he is capable of hating Albus is proof that he is evil, then the same must apply to Harry - for here, too, they mirror each other. At the end of OotP, Harry too feels violent hatred towards Albus for his implacable calm when Harry has just lost a beloved father-figure.
Some of Snape's actions are open to dark interpretation - it's possible that he saves Harry because Voldemort wants to kill him himself, for example. But many are very hard to interpret in any way other than that Snape's true allegiance is to the Order/Dumbledore.
He warned the Order that the children might be heading for the Ministry, thereby thwarting Voldemort's plans and bringing about the arrest of several prominent Death Eaters. And whether or not he knew Tom was trying to use Harry to get the prophecy, he certainly knew he'd been trying to lure Harry to the Department of Mysteries, and that alerting the Order would disrupt Voldemort's plans.
There was a delay of some hours between his first checking on Sirius and his warning the Order that the children might have gone to the DoM. Some people think he was leaving time for what Voldemort wanted to do, and then only pretending to alert the Order in order to maintain his cover with Albus. But if so, why warn the Order before Voldemort had finished?
We don't know when Snape found out that the children had gone to the Forest, or whether he searched the school first. The Slytherins in the Inquisatorial Squad will have told him what Hermione said about a secret weapon, and he knows there isn't one, so he'd know she was luring Umbridge somewhere: but not where.
His position vis-à-vis Umbridge was precarious so he wouldn't want to interfere if she had the situation under control; and if Hermione had the situation under control he wouldn't want to rescue Umbridge. And unless he actually thought of Thestrals he wouldn't know the children could get to London. No brooms were missing, the Floos were being watched, none of them could Apparate and the Knight Bus apparently can't get into the grounds (probably because it moves by Apparition). Even if he did think of Thestrals he didn't know the children had been conveniently showered with blood with which to lure them. His initial assumption when they didn't return would be that they'd got into trouble in the Forest. If he was Voldemort's he could just have gone on "assuming" they were still on school ground and not warned the Order - but instead he chose to thwart Voldemort's most important plans.
He almost certainly told Dumbledore about at least part of his Unbreakable Vow. And if Snape was really planning to off Dumbledore to serve Voldemort's interests, or even keeping his options open and considering killing Dumbledore to save his own skin, why make the job so much harder by forewarning him?
As he flees across the Hogwarts grounds, with Harry at his heels trying to cut or Crucio him, he stops and gives Harry a last hurried lesson on how to fight better.
Barty Crouch Jnr did teach Harry a few skills, but he was pretending to be someone else. Bellatrix at the Ministry advised Harry on how to cast Crucio, but she's a mad bint and she was gloating that what was needed - the desire to cause pain - was something Harry couldn't do.
Snape at that point has no position to maintain: the only way the Order will take him back is if he's truly loyal and can prove it conclusively, and if he isn't or can't there's no point pretending. He's sane, and he doesn't seem pleased at Harry's failure, but rather annoyed by it and trying to improve on it. There seems no reason for his stopping to teach Harry, in the middle of fleeing for his life, except that he badly wanted Harry to be able to fight better. And that is not the motive of a loyal Death Eater.
The reason which Snape gave for stopping one of the Death Eaters from Crucioing Harry was clearly bogus. It might be true that Voldemort wanted to deal with Harry himself, but there was nothing to stop Snape stunning and binding him and taking him to Tom as a present. Instead, Snape protected Harry from Crucio; did him no harm except to disarm him and to give him the magical equivalent of a slap when Harry accused him of being a coward; gave him a useful (if typically ill-mannered) final DADA lesson; and left him free to report what he'd seen, despite the fact that he should at least suspect Harry had been a witness to his killing Dumbledore.
If Draco realized that two brooms meant someone else had arrived with Albus, Snape surely should be able to work it out too. He knows Harry has an Invisibility Cloak and that Harry was very quick off the mark in pursuing him. He must suspect that Harry had been present, invisible, to witness Dumbledore's death and he certainly knows that Harry has seen that he is leading the Death Eaters rather than pursuing them. Harry's evidence may condemn him to death or to a life-sentence in Azkaban - yet he neither harms nor Obliviates Harry, only protects and teaches him, one more time.
And then, as Snape fled, Buckbeak tore at him, and Snape did nothing to defend himself. Even if he hadn't the energy for another Avada Kedavra you would think he would be able to do something to drop a flying predator the size of a horse which was trying to maul him, but he does nothing. The simplest explanation seems to be that he either didn't want to harm an innocent animal or he didn't want to upset Hagrid by hurting his pet - and these too are probably not the motives of a loyal Death Eater.
So here we have our man - stressed-out, ill-tempered, sarcastic and harsh, sometimes childish and often spiteful but also brave, protective, ethical, dutiful and self-sacrificing. He's certainly no angel - but his virtues compare well with the best that the light side has to offer, and his faults are arguably less than those of many other "good" characters. Being a bit petty and spiteful hardly compares with trying to murder someone for kicks, or covering up information which could save a child's life, just to save face.
I once characterized Snape to the writer duj as "Doing the Right Thing with exemplary courage whilst bitching about it the whole way" - to which she replied "That's our Snape, all right".