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But Snape is just nasty, right? (post-DH version)
N.B.: This essay has been somewhat re-worked in the light of new evidence presented in Deathly Hallows, but the original version can still be viewed here for comparison purposes, so you can see what I did and didn't get right. Page references refer to the UK hardback editions.
AS A SCHOOLBOY:
contents list
He is immersed in the Dark Arts:
JK Rowling did say in a Live Chat that young Snape loved Dark Magic and was "drawn to ... loathesome people and acts", but she didn't say whether he actually performed any loathesome acts himself, or was simply fascinated by those who did. In any case, book canon takes precedence over interviews, and the evidence for young!Snape being heavily into the Dark Arts in the actual books is very weak. Nor is there just an absence of evidence that he was very heavily into the Dark Arts: there is an actual presence of evidence that he wasn't, or at least that he wasn't generally considered to be, despite what Sirius says later.
Within the books, the information that young!Snape was fascinated by the Dark Arts comes only from Sirius. Lily, even when she is furious with Sev, does not accuse him of practising Dark Arts himself: only of associating with other Slytherins who do - and this is in fifth year, when if Sirius is to be believed Snape already had an established reputation as a Dark wizard in the making.
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. But we know that Snape was a real Death Eater, at least for some 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."
So, we know without doubt that Sirius is a biased source. We also know he at least claims Snape was heavily into Dark Arts - and yet he also says he's never heard even a rumour that Snape was a Death Eater. If Snape really had been famously Dark, it seems unlikely there wouldn't have been at least rumours that he was a Death Eater - even if he hadn't really been one.
Sirius says that "Snape knew more curses when he arrived at school than half the kids in seventh year". This is a profoundly 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 logical 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 even Umbridge admits that the fifth-year Potions class he later teaches is quite far ahead for their age, yet when they start Advanced Potion-Making in sixth year the recipes in it are new to them. Ergo, Advanced Potion-Making really is quite advanced.]
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 yet cast them effectively. And he knows Sectumsempra, which is probably also considered a curse.
[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.]
So that's three definites, all sanctioned by the school, three sort-ofs and a probable, for an Outstanding DADA student at the transition-point between sixth and seventh years. On this evidence, "knows more curses than half the seventh years" probably means "knows six curses". Indeed, we can assume fairly confidently that Severus didn't know any more than five or six curses, because if even the most Outstanding DADA student knows only seven curses, three of which he can't do, that means that if Severus had known more than six he would have known more than almost all the seventh years, not just half of them, and Sirius would have said so.
We also have to consider that the Marauders repeatedly hexed Snape, even by their own admission, and seem generally to have come out on top in their encounters (if they'd lost, Snape wouldn't have been so bitter about it, and James would have been able to produce a better excuse for attacking him than "he exists"). Evidently, they knew a lot of curses from an early age, too.
I suppose some people might interpret the scene in Deathly Hallows where a branch falls on young Petunia after she has mocked Snape as being proof that he was using curses even before he started at Hogwarts. But so far as I can see this scene is analogous to the ones where Harry unwittingly vanished the glass barrier at the zoo, or inflated his horrible aunt: a sudden involuntary, unconscious lash of wandless magic from a child who was angry and humiliated.
He may well have been taught hexes as quite a small child, though: whether or not he ever used them. Severus was born in 1960, in what looks as though it is probably the Manchester area. Out here in the real/Muggle world, from 1963 to 1965 that area was haunted by the paedophile serial killers known as the Moors Murderers, so Eileen Snape would have good reason to think that the Muggle world was dangerous and her son should be taught to defend himself.
What contemporary evidence is there that Snape was a Dark wizard? Lily accuses fifth-year Severus of associating with someone (Mulciber) who uses Dark magic, but not of using it himself. Severus is dismissive of whatever Mulciber tried to do, and says it was just a laugh, but we don't know whether Mulciber's spell was really bad or not. As an adult Death Eater Mulciber (a Mulciber, anyway) was famous for his use of Imperius, so it may be that he tried to Imperius Mary Macdonald - in which case, it was highly illegal, but how nasty he was being, and how appropriate or otherwise it was for Severus to play it down, depends on what he was trying to make her do - on which we have no information.
At any rate, whatever Mulciber did probably wasn't as bad as trying to feed somebody to a werewolf. Admittedly Lily doesn't know at that point quite how deadly what Sirius did was, but she won't allow Severus to tell her. And he, at least, knows that she is blaming him for hanging around with somebody who tried to pull a nasty prank, while she herself is associating with a would-be murderer. This presumably contributed to his tendency to play down whatever Mulciber did.
[It's not clear whether Mulciber was a bully anyway, in the sense of persecuting specific individuals, or whether he scattered his hexes broadcast. Nor do we know whether Mary Macdonald was an innocent victim or a formidable opponent against whom Mulciber had a legitimate grudge. Given how inept teenage boys are at expressing sexual interest, it's entirely possible that Mulciber tried to hex Mary because he fancied her. A friend of mine was mad keen on a girl at his school when he was about fifteen, and the only way he could think of to express his interest and admiration was to flick little pellets of rolled-up paper at her.]
Young!Snape writes long answers for his DADA OWL, suggesting his interest may be as much defensive as 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", which suggests that his evil reputation was not nearly as established as Sirius would later claim.
Of course, he may have developed a deep interest in the Dark Arts later, in sixth or seventh year. He had seen, after all, that the "Light" side were allowed to get away with attempted murder, so he would not see (we know from his comments to Lily that he did not see) that practising Dark Arts was an obviously more evil thing to do than bullying people or trying to feed them to a werewolf. But we can certainly dismiss Sirius's claim that James persecuted Snape because he was famous for being heavily into Dark Arts, since we can see James treating him with great cruelty in fifth year, at which point he clearly isn't regarded as a famously Dark wizard.
In fact, we see James begin to bully Severus on the train on their first day at Hogwarts, for no reason except that he wants to be in Slytherin. So Sirius's statement that James attacked Severus because he thought he was a Dark wizard is either a lie (because he attacked him when he knew nothing about him except that he wanted to be in Slytherin) or, if true, it means that James simply assumed without evidence that anybody who was in Slytherin was a Dark wizard (because he attacked him when he knew nothing about him except that he wanted to be in Slytherin). Sirius, coming from an apparently Dark family who were all Slytherins, may well sincerely believe the latter; but either way, it cannot be taken as strong evidence that Severus actually was Dark.
Then there is the issue of Sectumsempra. We do not know whether young!Snape invented it or merely learned it, but he certainly used it. How unpleasant a spell is it?
Sectumsempra means something like "always cutting" (so Snape may have either invented or learned it because it could be read as "Sever Forever"). We learn in DH that if it is used to actually cut a body part off, that body part can't be replaced. That could be taken to mean that Sectumsempra is an especially vicious curse which was specifically designed to "cut for always" - to amputate body parts in a way which prevents their regrowth. However, it seems from Remus's comments on George's injury that this is common to all amputations which result from a curse, of any kind:
'...there's no chance of replacing his ear, not when it's been cursed off --' [DH ch. #05; p. 64]
It's possible that Remus was going to say "...not when it's been cursed off with Sectumsempra", and was interrupted: but Molly then goes on to say:
'I can't make it grow back, not when it's been removed by Dark Magic.' [DH ch. #05; p. 66]
Molly's comment could be taken to mean that the spell which removed George's ear must be definitely Dark, although Remus's comment implies that any curse would do the same, and we know some curses are taught at Hogwarts. The definition of what is and isn't considered Dark Magic is never explained: it seems to be "a curse I don't approve of", and even "curse" has never been satisfactorialy defined.
[This is another topic for another day, but curses appear to be spells which are at least potentially harmful and which have especially strong effects, such as petrifying somebody until they are released. The term "curse" seems to be a measure of how risky a spell is, rather than how evil it is.]
It could be that when Molly said the injury was due to Dark Magic she meant something clearly bad, not the sort of curse which would ever be taught at Hogwarts, and Remus was going to say "...not when it's been cursed off with Sectumsempra". It could equally be that Remus meant what we heard him say - that any curse which caused an amputation would have the same effect, including the ones sanctioned by the school - and Molly was speaking sloppily and using "Dark Magic" as a synonym for "curse".
Some people have interpreted the fact that Snape used a "counter-curse" to heal Draco after Harry cut him as meaning that even simple, non-severing wounds made with Sectumsempra won't ever heal unless you know a specific and obscure counter-spell. The evidence in the books as to whether counter-curses and counter-jinxes are specific counters to specific spells is conflicting. On the one hand we have:
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]
On the other hand we find:
'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]
'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 although it did take Snape three passes with the "counter-curse" to heal Draco, his doing so is later 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, combined with the ease with which Molly stopped George's severed ear from bleeding (even if she couldn't regrow it), and the fact that the cut on James's face did not seem in any way remarkable, all suggest that aside from preventing regeneration of a severed part Sectumsempra produces simple, readily-healable cuts.
Nor is it the case that Sectumsempra results in abnormal scarring. Snape tells Draco that his wounds won't scar so long as they are treated promptly with dittany, which seems to be a standard wizarding treatment for wounds. Molly seems to close George's wound with little difficulty, and Draco's wounds are specifically contrasted with the unhealing scars which Bill suffers after being attacked by Greyback.
Harry looked over Hermione's shoulder and saw an unrecognisable face lying on Bill's pillow, so badly slashed and ripped that he looked grotesque. Madam Pomfrey was dabbing at his wounds with some harsh-smelling green ointment. Harry remembered how Snape had mended Malfoy's Sectumsempra wounds so easily with his wand. [cut] 'No, I don't think that Bill will be a true werewolf,' said Lupin, 'but that does not mean that there won't be some contamination. Those are cursed wounds. They are unlikely ever to heal fully,' [HBP ch. #29; p. 572]
The implication is that other than the no-regrowth-after-amputation aspect, Sectumsempra creates normal, readily healable cuts which are in contrast with Bill's cursed, abnormal wounds. And we see, from its effects when Harry used it on Draco, that it doesn't cut bone (or at least not at all easily), so it would be very unlikely that it ever would cause amputation. The severing of George's ear was a freak accident.
Snape's use of it against James, therefore, was equivalent to defending himself with a Muggle knife, except that it was a knife which he was able to project at a distance. Coming from a rough area, he may even have learned it (or invented it, if he did) precisely because he could use it to defend himself against Muggle attackers in a way which wouldn't leave obviously magical lesions.
So, we know that any "Dark" curse which can cause amputation will do so permanently, and this may well also be true of curses which are considered "Light" enough to be on the Hogwarts curriculum, such as Reductor. Nor is this even very noteworthy, because although regeneration of amputated parts may be possible in some cases, it certainly isn't the case that magic can always regenerate a lost part, unless it's been cursed off. The outgoing Care of Magical Creatures master, Professor Kettleburn, retired "in order to enjoy more time with his remaining limbs" and they had presumably been torn or bitten, not cursed off.
Furthermore, Harry, commenting on the ruin of his parents' house, says:
'Maybe it's like the injuries from Dark Magic and you can't repair the damage?' [DH ch. #17; p. 271]
and when Bill is injured by Greyback, Lupin comments:
'Those are cursed wounds. They are unlikely ever to heal fully,' [HBP ch. #29; p. 572]
This certainly sounds as though wounds caused by truly Dark magic shouldn't heal at all, or should scar badly. Yet even though Harry cut Draco very badly, Snape reckoned he shouldn't be scarred at all, and the method he recommended to prevent scarring was simply dittany, not some huge piece of magic. And Sirius never suggested Severus had scarred James for life - which you'd think he would comment on if he had - nor did Harry notice a scar in later photographs of his father.
Therefore, we cannot be certain that the fact that Sectumsempra is classed as a curse and can cause permanent amputation means that it is especially Dark. If anything, the fact that wounds caused by Sectumsempra but which stop short of amputation can be healed without leaving a mark is evidence that it isn't very Dark. The logic seems to be: not all curses are classed as Dark; all curses which can cause amputation are liable to cause permanent amputation; curses which can cause physical wounds and which are classed as Dark cause unhealable wounds or permanent scarring even from injuries which stop short of amputation; Sectumsempra causes permanent amputation but does not otherwise cause permanent scarring or unhealing wounds; Sectumsempra is a curse, but not a Dark curse.
We certainly cannot assume that Sectumsempra was invented in order to cause permanent amputation. Indeed, we can assume that it wasn't, since this seems to be a common feature of all curses which can cause traumatic injury, and therefore there was no reason to invent a special spell for it.
The name "always cutting", therefore, almost certainly doesn't mean "amputate permanently". It must have another meaning: possibly "the knife which needs no sharpening" - bearing in mind JK Rowling's propensity for bad puns based on British product-names (Spello-tape, Ethelred the Ever-Ready etc.) and the fact that Staysharp is a famous brand of kitchen knife.
We do know Sectumsempra equates to a knife rather than a sword or an axe. It's true that Harry thinks that it cuts Draco as if it was a sword, but that's because it cuts him in a long sweeping slash from a distance, rather than because of the nature of the injuries - that, or Harry has no idea what a sword would do. Harry swung at Draco with Sectumsempra in a panic, with all his considerable force. The resultant cuts are described as "deep" and they required three passes to heal them, which probably means they went right through skin and into muscle, but Draco was not disembowelled, beheaded or cut in half, nor so far as we can tell were any of his bones cloven or snapped, so the cuts were nothing like as deep as they would have been if made with a sword - unless it was a very light sword such as a foil, which would cause similar injuries to a knife anyway.
Initially Harry sees a lot of blood coming from Draco, but a little blood goes a long way, and when he gazes at the scene after Draco has been removed he sees water on the floor with some blots of blood floating in it - not a sea of blood. Draco was still able to walk as soon as he was healed, without being given Blood-Replenishing Potion, and he did not show any signs of physiological (as opposed to psychological) shock, which again suggests he had not lost a huge amount of blood, and his wounds were not terribly deep.
We also see Snape use a slicing hex on James in the bullying scene. It's not absolutely clear that this is Sectumsempra since he produces it non-verbally, but JKR's purpose in that scene was presumably to establish a link between Snape and slicing hexes which would later been seen to have been a clue to the identity of the Half-Blood Prince, and Lupin says Sectumsempra was always a speciality of Snape's - so it would be very strange if he just happened to be using a completely different hex here. And again, although it gashes James's cheek it doesn't slice his cheek right open, or damage bone, so here too the action of Sectumsempra is that of a fairly short-bladed knife - albeit a knife which could be projected several feet.
It has been suggested that Snape meant to do James a mortal injury with a slashing, sword-like spell and simply missed. But apart from the fact that we are told that he aimed straight at James, James was more or less in a line between Snape and Lily, so if Snape had been taking a great hack at James with a spell of which he was in uncertain control, he would have put Lily in danger. Since we know he will risk death or Azkaban rather than knowingly endanger Lily, we must assume that he was in complete control of his slicing hex and knew it, and that it did just what it was meant to do.
We do see Snape aim it at a Death Eater's wand-hand in an attempt to protect Remus, but we aren't told whether he thought it would amputate the hand, or just slash it badly enough to make the target drop his wand. Judging from its effects on Draco and on James it's been designed not to cut bone, and therefore as far as possible not to cause amputation. It was just a fluke that it caught George on one of the few bits of anatomy which can be detached without going through bone.
The fact that Sectumsempra is a knife-spell rather than a sword-spell raises the possibility that Snape initially used it as a tool, for chopping potion ingredients without bruising them or for sneakily snipping shoelaces, and only later designated it "for enemies". We don't know when he labelled it "for enemies", but the first time we see him use it - during the underpants incident - is after Sirius tried to feed him to Remus. After, that is, he knew that his "enemies" might actually murder him, and included a werewolf who was, when transformed, immune to most magic (otherwise Snape wouldn't have needed to be rescued, since he could use a wand and were-Remus couldn't). A weapon which caused actual physical cuts might be his only chance to save himself if were-Remus came after him again.
Much has been made of the fact that when Snape caught Harry using Sectumsempra he 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 seriously 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. On the other hand, if he was just being melodramatic when he said that a parchment found in Harry's possession was obviously Dark, he may equally well have been grandstanding when he accused Harry of using a Dark spell.
It's possible that both Sectumsempra and the Map may be Dark, because in DH we see Snape apparently sense the curse on the Peverell ring, which suggests he has some sort of psychic "feel" for such things. But if so that means that the Marauders too were working Dark Magic - and if they, who claimed to hate Dark Magic, nevertheless worked it without apparently intending to, the definition of Dark Magic must be something much more subtle and odd than "it's obviously evil", and probably has something to do with the way in which the spell operates.
So, we have a spell which acts as a small-to-medium-sized knife, which could be used as a tool as easily as a weapon. It is classed as a curse by Remus (a DADA teacher, so he knows his stuff), but that doesn't prove it's an evil spell, since some apparently quite benign spells (such as Petrificus totalus) are classed as curses, although they are taught at Hogwarts. It creates normal, physical cuts of shallow-to-middling depth, except that if it amputates a body-part it stays amputated. This last feature seems be common to all curses, rather than a special design feature. The fact that, amputation aside, Sectumsempra does not cause permanent scarring suggests that it probably should not be classed as truly Dark. And it doesn't cut bone even when used with some force, which suggests that it's actually been designed with a built-in safety factor to prevent it from causing amputation.
On the face of it, it seems no nastier than the Reductor curse, which also could be either a tool or a weapon, which probably also causes permanent amputation, and which is on the curriculum. The only evidence that it is considered Dark Magic depends on comments by Snape himself which apply equally to the Marauder's Map.
The evidence that Snape actually invented Sectumsempra is that the name can be rendered as "Sever Forever" (that is, as a play on the name Severus); that it was written down in his Potions book and that during the running battle at the end of HBP he accuses Harry of using his own spells, plural, against him when Harry tries to use both Sectumsempra and Levicorpus - although it is Levicorpus which prompts this accusation.
'You dare use my own spells against me, Potter? It was I who invented them' [HBP ch. #24; p. 491]
On the other hand, although it is written down in his book it seems to have no workings-out or instructions with it, as his other spells do. Harry certainly seems to assume that it was a spell which the Half-Blood Prince had learned, rather than one he had invented: he may or may not have been right about that, but it raises the possibility that JKR herself intended it to be a spell Snape had got from elsewhere.
What had the Prince been thinking to copy such a spell into his book? [HBP ch. #24; p. 491]
When we see Snape use what appears to be Sectumsempra to defend himself from James during the underpants incident, he casts it non-verbally, so the Marauders don't hear what the spell is called, and yet in DH Lupin identifies a spell cast by Snape as Sectumsempra and says it was always a speciality of his. It sounds as if Sectumsempra is known to Lupin in a general way: hence, it may be a spell which Snape used rather than one which he invented.
That's not absolute proof, of course, because Lupin knew Levicorpus by name yet didn't know Snape had invented it, or didn't admit to knowing. But it does mean that it's perfectly possible Snape didn't invent Sectumsempra: just took it up because of the coincidence of the name.
If he didn't invent it, why would he accuse Harry of using spells of his own invention, plural, against him? Perhaps because even though he got it from elsewhere he had adapted it for his own use. Or perhaps in the heat of the moment he is speaking loosely. He knows Harry tried to use Levicorpus (although we don't know how he knows, since it's a non-verbal spell - perhaps Harry's lips move!) against him, and that's a spell which he invented. He knows Harry tried to use Sectumsempra against him, and it's certainly his in the sense of being his signature-spell and coming from his notes. He almost certainly knows Harry used his Langlock spell on Filch (twice). People don't always speak 100% accurately, and perhaps 'You dare use my own spells against me, Potter?' was just easier to say than 'You dare use one of my own spells against me, and use another of them against Filch, and use a spell which, even though I didn't invent it, I still regard as mine against me, Potter?'
Whether he invented it or not, it's not a particularly bad spell, and the spells which do have workings-out, which we definitely know he invented, are all humorous and fairly harmless: rather less spiteful or dangerous, in fact, than a lot of the Twins' bright ideas. Indeed, the fact that a book which is full of young!Snape's home-made spells and notations contains only one mildly nasty curse, which he may well not have invented, is pretty good evidence that he wasn't particularly Dark or unpleasant. You would expect that a serious Dark wizard, or even Dark wizard wannabe, would come up with something a bit more wicked than making toenails grow.
Furthermore, in his tirade to Harry at the end of HBP Snape says that James used his own spells, plural, against him, and we certainly see James use Levicorpus. If he's speaking accurately then either Snape's spells were so mild that James, who was supposed to hate Dark Magic with such a passion, regarded them as acceptable for his own use - or Snape's spells were Dark and James was a hypocrite who also used Dark Magic when it suited him.
So, other than an off-the-cuff comment by JKR during a Live Chat, the evidence that young!Snape was heavilly involved in Dark Arts rests on his use of Sectumsempra, which is itself not an especially bad spell, and on the word of a biased source. Other equally good, contemporary evidence - the nature of the spells which we know for certain he invented and which James found acceptable for his own use, James's failure to come up with a good excuse for persecuting him, and the fact that Lily only accuses him of having friends who are into Dark Arts, not of being into them himself - 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.
We know that young!Snape used Sectumsempra, whether or not he invented it, and we know it is classed as a curse, and like any curse if it causes amputation, that amputation will be permanent. That aside, it functions as a simple kife, which could be either weapon or tool. We don't know when he designated it "for enemies", but we do know that the first time we see him use it was after his life was threatened by someone immune to magic.
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 boys of that age; and indeed we are shown that first-year Harry shares Snape's interest, and expects that Dudley would too.
Even Dudley, who never read anything, would have been wild to get his hands on some of these. Hagrid almost had to drag Harry away from Curses and Countercurses (Bewitch Your Friends and Befuddle Your Enemies with the Latest Revenges: Hair Loss, Jelly-Legs, Tongue-Tying and much, much more) by Professor Vindictus Viridian. [PS ch. #05; p. 62]
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 housemates, 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 by the end of Deathly Hallows Harry has cast Unforgivables six times, once with partial success and five times with full success, plus a seventh attempt (casting Crucio on Snape), which was interrupted, 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.
Later, we see adult!Snape complain to Bellatrix that Dumbledore would never give him the Defence Against the Dark Arts post in case he was tempted back into his "old ways". JK Rowling said something similar in an interview, but this was before HBP and its revelation about the cursed DADA post came out, so she may well just have been planting a red herring. The fact that Dumbledore gives Snape the DADA post once he knows that, one way or another, Snape will be leaving the job in a year anyway suggests that his reasons for not giving Snape the job beforehand were to do with the curse, not because he didn't trust him. The last thing he would want to do would be to tempt Snape towards the Dark Arts just as he himself was about to die, leaving the safety of Hogwarts in Snape's hands; so it's unlikely he really thought Snape would be corrupted by the post.
The fact that Voldemort sent Snape to apply for the DADA post in the first place suggests that the curse would not apply to someone who was acting as his loyal agent, or at least, Dumbledore would reasonably assume that it wouldn't (although in fact Barty Jnr ended up Kissed). So Dumbledore's reasons for not giving Snape the post were two-fold. He didn't want Snape to suffer what might be severe consequences as a result of the curse, but he also didn't want Snape's true loyalties to be revealed, since he would expect that if Snape took the DADA post and then incurred the curse, that would tell Voldemort that Snape wasn't his man.
On the other hand, Bellatrix must find the idea of Snape as a Dark Arts practitioner to be feasible, if his explanation to her as to why he was never given the DADA post was to have any credibility. So he must have had at least some reputation for being into the Dark Arts when he was a Death Eater, or at least he didn't actually have a reputation for not being.
He is a racist:
When we see child!Severus interacting with Lily and Petunia, he tells Tuney he wouldn't spy on her because she's a Muggle - but this is a reasonable observation, since the very reason he's been spying on Lily is because she's not a Muggle, but somebody like himself. We would not think that the only Francophone child in a neighbourhood was racist for being preferentially interested in meeting another French child - somebody they could really talk to, who would understand them - for the first time. And he just says "You're a Muggle", albeit in a spiteful tone - he doesn't say "just a Muggle" or "only a Muggle".
It's obvious that in this scene he thinks being a Muggle is less interesting/worthy than not being, but no more so than you'd expect given that Muggles are defined as people who lack the skills in which he is interested. There's no suggestion he regards them as inferior in any other way. Later on when he is angry about Tuney he says "She's only a -" and then stops himself, and the next word might have been going to be "Muggle" - but it could equally-well have been "spiteful little cow".
Even if the next word was going to be "Muggle", there's no indication that he was any more racist than Hagrid, who calls Vernon "a great Muggle" (twice), and describes the Dursleys collectively as 'a family o' the biggest Muggles I ever laid eyes on.' Hagrid, clearly, means something rather more insulting than just "person without magic", but nobody seems to hold it against him.
When Lily asks Severus if being Muggle-born will make a difference to her at Hogwarts he hesitates for a moment, which suggests he's been taught some prejudice, or at least told that it exists - but he assures her that it won't, so if he's been raised with prejudices he's trying to overcome them. He assumes automatically that he will be in Slytherin, which means either his wizard family are Slytherins or he has a personal passion for Slytherin, yet he hopes Muggle-born Lily will be in Slytherin too - so obviously he has not been indoctrinated with any "Slytherin is the house for the pure of blood" agenda. Indeed, since his mother married a Muggle, it seems unlikely she was especially prejudiced against Muggles (even if she went off her husband in the end), or that she would fill her half-blooded son with pure-blood prejudices.
So, the idea that he was racist as a pre-teen rests on the fact that he spied preferrentially on a magical child rather than a non-magical one, that he thought not having magic was less interesting than having it and that he was aware of the existence of prejudice against Muggle-borns, although not of its true extent.
At the end of fifth year, Lily accuses Severus of calling all Muggle-borns Mudbloods, yet in the bullying scene, James does not justify his attack on young!Snape on the grounds of his racism, even though that would have been a perfect excuse to offer to Muggle-born Lily. All James could come up with was "he exists", at a point at which a good excuse would have seriously improved his chances with Lily. This suggests Snape was not a notorious racist, and was probably just using bad language he had heard from his housemates, who we know included many future Voldemort supporters.
By that point, if Lily is to be believed, he does often use racist language - yet he continues to be best friends with Lily, and we have not seen her accuse him of being racist before. She seems to be assuming that he is a genuine racist who makes an exception for her, since she says to him "But you call everyone of my birth Mudblood, Severus. Why should I be any different?" But since she refuses to allow him to answer, we don't know whether his reply would have been the essentially racist "But you're special, you're not like the rest", or "My mates in Slytherin all say it and I've just got into a bad habit", or even "I only say that because my housemates would batter me if I didn't: I'm in enough danger already because I hang around with you". He is, after all, surrounded by junior Death Eater wannabees who know where he sleeps.
Calling Muggle-borns in general "Mudblood" could even be a bad-taste joke on his part. He made a joke out of his own half-blood status, after all, and Lily at least is well aware that he's a half-blood. It's not clear whether, by calling himself the Half-Blood Prince, he's saying "I may be only a half-blood, but at least I'm half a Prince" - or whether the Princes were pure-blood fanatics, and by proclaiming his half-blood status he's sticking two fingers up to them.
The fact that he had friends who were Death Eater wannabees, and later became one himself, might suggest that he shared their racist views: except that JK Rowling has said at interview that one of the reasons Snape joined the Death Eaters at all was because he thought doing so would impress Lily. That suggests he really wasn't aware of the full extent of their racist views. And indeed, given that he's a half-blood, if the Death Eaters wanted to recruit him they would probably play down the racist angle.
So why did he call Lily a "filthy little Mudblood" during the bullying scene - which is, of course, much more racist-sounding than just "Mudblood" on its own?
To begin with, he fancies Lily and he knows James does too, so he wants to impress her and instead he has just been made a fool of in front of her, by a rival for her affections. 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.
Then, Lily smiled for a moment when she saw him hung up with his underpants on display. Possibly he had humorous legs, but even so it was insensitive of her to smile when he was in so much misery and humiliation, and it would have made him feel much worse and much more dishonoured in her eyes. He had reason to be angry with her, especially as she had been a witness to the whole of the bullying incident and yet hadn't intervened at once.
Some people take the fact that Lily's attention was on James rather than on Severus as proof that she was flirting with James over her supposed best friend's misery. Personally I think it's understandable she was watching James, since he was the one who was armed and threatening to hex her, but nevertheless JK Rowling has hinted that she was flirting with James in that scene, even if only subconsciously. That, again, would make Severus angry and bitter - at least as justifiably angry and bitter as Ron and Hermione were when they were hexing and insulting each other.
It is quite possible that Severus's drawing James's attention back onto himself by shouting at him, and disowning Lily by insulting her to James, was an attempt to get Lily out of a risky situation - since James had just threatened to hex her. Severus knows that Sirius, at least, is a potential killer, and he seems to believe - rightly or wrongly - that James was in on the attempt to kill him, until he got cold feet. So as far as he is concerned, Lily is being threatened with violence by someone she refuses to believe is truly dangerous despite his warnings, so he will be angry about her taking stupid risks and ignoring his warnings, as well as protective of her.
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 must have terrified him.
He hangs around with future Death Eaters:
Given his precarious position in Slytherin 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 Slytherin 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 1960, so Lucius was more than five years older than Severus, and Bellatrix was born probably in autumn 1953 (see accompanying essay on Birthdates in the Harry Potter universe). 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 if they did have such a relationship, Snape would have been so much the younger that even at best there would always be a strong suspicion of exploitation.
As for Avery and Mulciber, we know Severus hung around with them, and like Remus he made excuses for his mates' dodgy behaviour; but they probably weren't very close friends, since we know the Marauders were still persecuting him four to one after he started hanging around with them. Lily accuses him of wanting to join Voldemort like his "precious little Death Eater friends" - but she doesn't allow him to answer, so we don't really get to know if she was right at that point.
If we accept JKR's interview comment that Snape joined the Death Eaters partly to make himself look cool, thinking that that would impress Lily, then we must assume from this that he didn't know how murderous their pure-blood agenda really was. Presumably they played it down because he was a half-blood - and they must have known that he was, since his parents' marriage had been in the Daily Prophet. And indeed, JKR has said that the Death Eaters actually tried to recruit both Lily and James when they left school, which argues that at that stage the Death Eaters really weren't officially prejudiced against Muggle-borns, or certainly not in a way which appeared to be life-threatening.
We are told that, for years, young Regulus (who was about two years younger than Snape) "talked of the Dark Lord, who was going to bring the wizards out of hiding to rule the Muggles and the Muggle-borns". So there was an awareness that Voldemort's plans would result in the Muggle-borns possibly being second-class citizens, but also that he meant to make magic publicly known and acceptable to Muggles, which would have advantages for Muggle-borns as well as pure-bloods. And there's no suggestion Regulus thought mass slaughter was on the agenda, so presumably Severus didn't either.
And in any case - who in Slytherin at that time didn't hand around with future Death Eaters? There seem to have been so many that it would have been extremely difficult for any Slytherin not to hang around with future Death Eaters.
He spies on the Marauders and tries to get them expelled:
The bullying which we see young!Snape suffer is particularly psychologically nasty, and the nickname "Snivellus" was established on their very first day at Hogwarts, i.e. the bullying has probably been going on for the whole of Snape's schooldays. The choice of nickname also implies that the Marauders can make Severus cry, and enjoy doing so - but that could be spurious, since they gave him the nickname on the train to mock him for sticking up for himself.
Either way, 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." The incident on the train on their first day was of a piece with that - James made an unprovoked, sneering remark about Severus, and then when he responded in kind James escalated to outright jeering, again saying, in effect, "I can say or do whatever I like to you and you're not allowed to defend yourself," and that has evidently been James's attitude from first to last. 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.
He goes on hexing James even after James saved his life:
We can see from the order of events in Deathly Hallows that the Marauders, including James, went on persecuting Snape with great cruelty even after Sirius had nearly murdered him, and James had saved him. This shows almost unbelievable shallowness on their parts - the shock of having nearly killed a classmate evidently had no effect on their behaviour at all, nor did the gravity of what they had done register with them. Clearly, Snape was right to think that James saved him to protect the Marauders, not out of any concern for him.
We know that the hex-war between James and Severus continued into seventh year, and Sirius and Remus claim that Snape was the aggressor by that point. It's true that 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.
AS A YOUNG MAN:
He is a Death Eater:
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.
According to JK Rowling, Snape was attracted to "the dark side", but he was attracted not because he was dark himself but because he was insecure, and thought that "the dark side" was cool and would give him something to belong to.
Given his time over again he would not have become a Death Eater, but like many insecure, vulnerable people (like Wormtail) he craved membership of something big and powerful, something impressive. He wanted Lily and he wanted Mulciber too. He never really understood Lily's aversion; he was so blinded by his attraction to the dark side he thought she would find him impressive if he became a real Death Eater. [JK Rowling, Bloomsbury Live Chat, 30th July 2007]
Or, as libertyelyot put it: "...he was no more evil in his youth than the lads who take up wearing eyeliner and listening to Nine Inch Nails because their souls are so dark, man. But because he had nobody to talk to about anything, it all went so much further than he ever intended."
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. If we are to believe JKR's statement that Snape joined the Death Eaters partly because he thought it would impress Lily, then they must have played down the racist angle very much, until he was Marked and it was too late for him to get out.
Sirius in fact says in OotP that 'there were quite a few people, before Voldemort showed his true colours, who thought he had the right idea about things ... they got cold feet when they saw what he was prepared to do to get power, though.' The context suggests that what Sirius means is that people like his family (that is, well-informed Dark pure-bloods at the heart of wizarding society) knew Voldemort wanted to exclude Muggle-borns from wizarding society, or at least from positions of power in wizarding society, but didn't know he meant to kill them.
Indeed, from what Kreacher says, Regulus understood Voldemort to be going to overturn the Statute of Secrecy, bring wizardry into the open and have pure-bloods at the top of the tree, ruling over Muggles and Muggle-borns, but with no suggestion that Muggles and Muggle-borns who were willing to be ruled over might still be persecuted or killed. It seems unlikely that a marginalised half-blood teenager would have been better-informed than the Blacks, so we can assume that Severus at least didn't know in advance that the Death Eaters were wholesale killers, or that their racial theories were actually murderous.
Also, prior to Shacklebolt's appointment as Minister, British Wizarding society routinely subjected 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, was 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 circa January-March 1980. We know Snape was a true Death Eater at that time, that he defected to Dumbledore after the Potters were targeted, and that the Potters went into hiding because of Snape's warning. When did the Potters go into hiding?
In the scene in The Three Broomsticks in PoA, Fudge says one of Dumbledore's "useful spies" warned him that You-Know-Who was after the Potters. Dumbledore alerted James and Lily at once and advised them to go into hiding, and recommended the Fidelius Charm. They performed the Fidelius, and "barely a week" later they were betrayed by their Secret Keeper. This betrayal presumably coincides with their death, and it doesn't sound as if there was a very long delay between the warning and the casting of the Fidelius, either: so on the face of it this makes it seem that Snape defected and passed on his warning only about ten days before the Potters were killed, and they were only in hiding for a week.
On the other hand, Sirius accuses Peter of passing information for a year before the Potters died, i.e. since October/November 1980, and that information probably came from Snape, the Order's spy. Sirius didn't know at the time that Peter was the year-long leak (assuming that he was), and evidently Snape didn't either - he was desperate to protect Lily, so if he'd known who the leak was, he would have said - but Sirius knew there was a leak. It's possible Snape only became the Order's spy much later and then told them "there has been a leak going on for n months which you didn't know about". But JKR said at one point that the Potters already knew they might have to go into hiding by Harry's christening:
When Harry was born, it was at the very height of Voldemort fever last time so his christening was a very hurried, quiet affair with just Sirius, just the best friend. At that point it looked as if the Potters would have to go into hiding so obviously they could not do the big christening thing and invite lots of people. [JK Rowling, Edinburgh Book Festival, 15th August 2004]
The average age for a baby being christened in Britain seems to be about seven months (judging from a quick survey of fourteen examples mentioned on Google!), so if she means that the christening was "hurried" in the sense of being rushed into that means it was prior to February 1981, and Snape had already warned them about the security leak and the threat from Voldemort by that point.
In addition, Dumbledore tells the Wizengamot that Snape turned spy "at great personal risk", which sounds as if he'd been spying for a substantial period before Voldemort fell.
It's possible that Snape himself was the Order's year-long leak - that he defected about a year before Voldemort fell, and that then, as later, Dumbledore fed titbits of information about the Order to Tom via Snape to make him look convincing. As far as we know Tom knew from the first that Dumbledore thought Snape was working for him, Dumbledore, so he would have expected Snape to come up with at least some information to prove that he had "fooled" Albus.
Even more significantly, we see the letter which Lily wrote to Sirius following Harry's first birthday party. The Potters were certainly already in hiding when this letter was written. If Snape only defected in October 1981, and the Potters only went into hiding at the point that the Fidelius was cast, the letter would have to have been written in the last week of October 1981.
It certainly sounds as if the birthday party happened not long before the letter was written. It is unlikely that Harry's party was held months after his birthday, and Lily says that "James is getting a bit frustrated shut up here, he tries not to show it but I can tell -- also, Dumbledore's still got his Invisibility Cloak, so no chance of a little excursion", which suggests that they have been "shut up" for considerably more than a week. Nor does she sound as nervous and excited as you would expect if they had been forced to flee and go under Fidelius only days beforehand.
Almost everything about the letter indicates that it was written in August 1981, shortly after Harry's birthday. The only thing about it which might indicate that it really was written in the last week of October is that Dumbledore will later tell Harry, in the King's Cross scene, that he borrowed the Cloak only a matter of days before James's death; but we must assume that he is confused (he was already a hundred years old when he borrowed that Cloak, after all), or outright lying, because even if by some strange chance the letter had been written in late October, Lily would hardly complain that Albus "still" had the Cloak if he had borrowed it only days beforehand. Therefore, we know that Dumbledore's statement about when he borrowed the Cloak is untrue, and we can discount it as evidence of when the letter was written. Everything else points to it having been written in August.
Lily also says that "Wormy was here last weekend, I thought he seemed down, but that was probably the news about the McKinnons; I cried all evening when I heard." James and Lily seem to have regular contact with the Order, since they have at least heard from Sirius (they knew he wouldn't be able to make the birthday party due to Order business) and Peter in the last couple of weeks. This means that they would have heard about the murder of the McKinnons soon after it happened - and they expect it still to be fresh enough news to Peter that he would still be depressed about it as recently as the previous weekend. Ergo, the letter was written not long after the McKinnons died.
In OotP Moody shows Harry a group photograph of the Order and says "that's Marlene McKinnon, she was killed two weeks after this was taken, they got her whole family." There is no suggestion that it took a long time to discover that the McKinnons were dead, so we can assume that this photograph was taken three or four weeks before Lily's letter was written. If the letter was written after the Potters went under Fidelius, then the photograph was taken around the start of October 1981.
[We also know that James and Lily attended an Order meeting three or four weeks before the letter was written, since they too are in the photograph: so either they didn't go into hiding until after the photograph was taken, perhaps because of the McKinnons' deaths, or a trip to an Order meeting was one of the "excursions" for which they needed the Cloak. This gives us at least a hint that Dumbledore borrowed it towards the end of July rather than earlier, if in fact the letter was written soon after Harry's birthday.]
Moody then goes on to list four other Order members - Benjy Fenwick, Edgar Bones (and family), Gideon Prewett (and brother), Dorcas Meadowes (killed by Voldemort himself) - who were all killed in Vold War One, plus Caradoc Dearborn, who vanished six months after the photo' was taken, and whose body was never found. If Lily's letter was written at the end of October, that would mean either that Voldemort's forces managed to kill four Order members in the last month before his downfall, or that some of these people were killed after Voldemort's downfall.
We know that Antonin Dolohov was convicted of being one of the five Death Eaters who murdered the Prewett brothers - it's in OotP, in the Prophet's report on his escape from Azkaban. In the Pensieve scene in GoF, Crouch Snr tells Karkaroff that Dolohov was captured and Rosier killed shortly after Karkaroff's own capture, which means the capture of Dolohov and the death of Rosier happened at about the same time - that is, they both happened soon after Karkaroff was captured. This in turn means that Rosier's death cannot have occurred more than a few weeks before the murder of the Prewetts, and more likely happened after it (because Rosier's death happened at about the same time Dolohov was captured, and Dolohov must have been captured after he killed the Prewetts).
In GoF, Sirius says that Rosier and Wilkes were "killed by Aurors the year before Voldemort fell", which doesn't sound as if Rosier was killed only a few weeks before that fall, and almost certainly means that the Prewetts were killed before the Potters. Karkaroff offers Crouch evidence against Travers who "helped murder the McKinnons", which implies that his own capture was after the death of the McKinnons, since he doesn't seem to know anything about events after his own arrest. And we know that his capture occurred before the death of Rosier, which in turn was before the fall of Voldemort.
It is of course possible that Sirius was speaking extremely loosely when he said that Rosier died the year before Voldemort's fall, and that the murders of the Prewetts and McKinnons, of Dorcas Meadowes and probably of Bones and Fenwick, Karkaroff's capture, Dolohov's capture and Rosier's death all occurred in October 1981. But if Order members have been dropping like flies in recent weeks, why would Lily single out the McKinnons as a cause of sorrow, rather than saying "all the deaths in the Order"? And why would Moody specify that the McKinnons were killed only two weeks after the photograph was taken, as if it was, literally, remarkable, and yet not say that Meadowes and the Prewetts were killed only a week or two after that?
What is more, Moody says that in the Order photograph Sirius "still" had long hair. It's unlikely he's referring to the long hair Sirius grew in Azkaban, since he didn't learn about that until twelve years later - if that was what he'd meant, he'd have said "he had short hair in those days". So he's refering to a sartorial choice which Sirius made to grow his hair long, prior to his arrest.
Since Sirius was arrested the day after the Potters were killed, if the photograph had been taken only three or four weeks before their deaths Sirius would only have had three or four weeks to grow his hair long. He could only have done so by magic, and it would have been a flash-in-the-pan, something seen only for a few days or weeks. And again, Moody probably wouldn't phrase it as Sirius "still" having short hair in the photo', implying that long-haired was his normal or characteristic state.
To allow Sirius time to have grown his hair and for Moody to have seen that as his normal state, we need to set the taking of the photograph as early as we can get it and still have Lily find out about the Mackinnons' round about the time of Harry's birthday party, or just before it. That sets the photograph in mid July 1981, and confirms that Lily wrote the letter soon after Harry's birthday, by which point the Potters were already in hiding..
When combined with Lily's implication that the Potters had been cooped up for a long time before she wrote the letter, the mention of Harry's birthday party as a recent event, Sirius's knowledge of how long there had been a leak in the Order and JKR's interview statement that the Potters already knew they might have to go into hiding before Harry's christening, which was rushed into early, it seems clear that they went into hiding in two stages. Snape defected and warned them that Voldemort was interested in them and they confined themselves to the house, but continued to receive visitors fairly freely. This happened at some point long enough before Harry's first birthday that by August 1981 James was thoroughly fed up with the restrictions. Then in late October 1981 Snape relayed the news that Voldemort was about to strike, the Fidelius Charm was cast and the Potters went under deep cover, only to be betrayed and killed a week later.
That would mean that the murder of the McKinnons occurred in late July or early August 1981. The group photograph of the Order was taken in mid July 1981, Dumbledore borrowed the Cloak shortly afterwards, and the various other killings and captures which Moody and Crouch listed occurred between then and the end of October - except for the disappearance of Caradoc Dearborn, who disappeared some time in January 1982. Given that his body was never found, one wonders whether he really was abducted and killed by stray Death Eaters, had some personal and private reason to disappear, or simply got smashed at a New Year party and fell in the river. Or perhaps he was the long-term spy in the Order, and chose to vanish.
This interpretation - that Snape's defection happened well before Harry's first birthday - is supported by the description of the scene in which we see Snape come to Dumbledore and ask him to save Lily. We are told that it is very windy and the trees around them are bare, but there are still leaves blowing in the wind. Unless this is extremely freak weather, this is not the same autumn that the Potters died, because they died at the end of October, and late October and early November is the peak time in Scotland for viewing the autumn leaves, still on the trees. If the trees are bare it's no earlier than mid November, and if there are still leaves blowing around which haven't decayed to mulch, it's probably not later than mid January. So we know Snape ceased to be a true Death Eater no later than January 1981.
On the other hand, we can reasonably assume that the defection scene is not taking place in early 1980, immediately after the prophecy was given - even if the prophecy was made as early as January. We assume this because Snape refers to Lily's son, and speaks as if the baby could be killed and yet the mother spared. It's possible that the sex of wizarding children is announced soon after conception, and that he is thinking of Voldemort hexing the child in the womb without killing the mother, in which case this scene could be taking place in January 1980. On the whole, though, it sounds more as if Harry is already born - especially since it was only after Harry's birth that Voldemort could be sure he would be born in late July rather than early August, and fitted the prophecy. Therefore, Snape's defection occurred between mid November 1980 and mid January 1981.
We don't know how long Snape had been a Death Eater before hearing the prophecy. It could have been four years - or four days. We know Lily accuses him of being eager to join Voldemort at the end of fifth year, almost four years before the prophecy was made: but since she doesn't really give him a chance to comment, we don't know if she was right or not, or what effect their break-up might have had on his views.
It's probably safe to assume, though, that he wasn't already a Death Eater at that point, unless he'd only joined a few days beforehand, because if he is one Lily clearly doesn't know it, and JKR said he joined partly because he hoped it would impress her. So if he had joined while they were still friends, he would have told her. The earliest he could have become a Death Eater, then, was mid June 1976.
So, we only definitely know Snape was a true Death Eater for about eight months, from March to November 1980, but it is likely that he was one for at least some months beforehand, and possibly for a lot longer. The longest he could have been a Death Eater for was four and a half years, from June 1976 to January 1981.
If we assume that his joining the Death Eaters partly in order to impress Lily means that he was hoping to win her back, he probably did join prior to her marriage. Unless she married pregnant (unlikely, as one would assume wizards have very good contraception) or Harry was premature, she must have married prior to November 1979, and Snape became a Death Eater no later than October 1979 - more probably somewhat earlier, since it's unlikely he was still trying to impress Lily only weeks before her marriage.
The idea that Snape joined in 1979 would tie in with his statement about having sixteen-years'-worth of information to give to Voldemort on his resurrection in June 1995. It may be, however, that he joined earlier than 1979, and really did begin to spy on Albus in 1979, or very early in 1980 (fifteen and a half years before Voldemort's rebirth).
The most likely scenario is probably that he joined some time in early 1979, and that he was at the Hog's Head in (probably) January 1980 because Trelawney was right, he really was applying for a job in the hopes that he would then be able to spy on Dumbledore. In order to sound good to Bellatrix he conflated sixteen and a half years as (as far as she knows) a Death Eater, fifteen and a half years attempting to spy on Dumbledore and fifteen years actually doing so into sixteen years of spying.
The fact that we don't know what Snape did between leaving school in summer 1978 and hearing the prophecy in early 1980 doesn't prove he was working for Voldemort since he left school. 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.
We also do not know whether Snape was a keen Death Eater until the danger to Lily changed his mind; or whether, like Regulus, he developed doubts soon after joining, but it took a threat to a loved-one to move 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."
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 Dumbledore chides Snape about being squeamish about sending Harry to his death, he asks him how many men and women he has watched die - not how many he has killed. [And he replies that latterly - since he ceased to be a true Death Eater, presumably - he has only watched die those whom he was unable to save, thus confirming that he is not somebody who enjoys violence, or even is indifferent to it.]
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 and deaths, but he may never have taken an active part: he certainly didn't make a habit of doing so, and it is strongly hinted that he never killed until that night on the Astronomy Tower. And we know that latterly, as a spy, he has done his best to stay out of direct physical action, and to save the victims if he could - 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.
On the contrary, we see that Dumbledore feels obliged to order him to play his part convincingly in the chase after the false Harries. "If you blow your cover you'll be killed" isn't enough to make him obey, either: the lever Dumbledore uses is that he needs to maintain Voldemort's confidence in order to protect Hogwarts from the Carrows. Evidently, Dumbledore believes that Snape values the students' lives more than his own. And even so, Snape still disobeys - to protect Lupin.
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 eight months, more probably for about twenty months, 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.
He is obsessive about Lily:
[His relationship with Lily of course spans his entire life from age nine until death, but I've put it in the "young man" category because the accidental betrayal and Lily's death is so central to the plot.]
Many people think that Snape's attitude to Lily was stalkerish, and this is in part because child!Snape is described as watching Lily with an expression which is "greedy". He is unlikely to have felt anything like sexual desire when he was only nine or ten, though, so presumably his greed was for the company of another wizard child, or to have something beautiful in his grey life - or just to be able to say "I know something you don't know". He does rather like to gloat.
Once they have become friends, he is again described as watching her greedily. Since he's too young for sexual desire his infatuation is presumably of the platonic kind - intense feelings of friendship and admiration. That he already had such strong feelings for Lily, before he was old enough for sex to be involved, in turn means that sex was probably not the main driving force behind his later passion for her. It was not the case that he fancied her, and then he loved her - he loved her, and then he fancied her.
In any case, JK Rowling seems to have a rather negative attitude to desire. Child!Snape's greed is certainly no more worrying or dark than teenage!Harry's large scaly chest-monster which flares up when he sees Ginny - who isn't even his girlfriend yet - kissing another boy. Ron and Hermione tear at each other, Albus's desire for Gellert leads to the death of his sister, and Bellatrix's desire for Tom is mania.
Child!Snape's approach to Lily is awkward but after telling her she's a witch, he remembers to tell her that that's an OK thing to be; and he reassures her that being a Muggle-born won't make any difference to her status, even though his hesitation suggests that he knows it will. He is kind to her from the first, then, and admires her from the first, even though he is tongue-tied about saying so: he starts to say 'You're not going to end up in Azkaban. You're too --' and then stops and flushes red.
He becomes spiteful to Petunia only after she is spiteful to him, and there is nothing to suggest that his dropping the branch on her was voluntary: it appears to be the same sort of spasm of involuntary magic which Harry used to vanish the glass from the snake's cage or to inflate his aunt, when he too was upset and angry. Lily, however, assumes that he must have done it deliberately, and blames him, even though she herself has wandless magic and must know how it works. Unless, of course, Lily is always in control of her magic, and of herself.
We do not know whether it was Severus or Lily who opened Petunia's letter from Dumbledore: only that (if Lily is telling the truth) he was the one who saw the envelope first. If Lily's comment is to be believed, Severus's main interest in the letter was not nosiness about Petunia, and certainly not malice, but honest curiosity about the workings of the wizarding world, and how a letter sent via the Muggle post office could end up at Hogwarts.
Nor do we know whether he read it or not: only that Lily did. They were presumably both in Petunia's room, but Severus could hardly have been in there without Lily's connivance: and since she went on to read the letter she was certainly not going "How dare you invade my sister's bedroom?" It was certainly Lily who was stupid enough to tell Tuney she'd read her private correspondence. Yet she blames Severus for all of it.
When Lily says that Tuney hates her Severus says "So what?" which seems rather callous - but since his family appear to be at each other's throats all the time he may genuinely not see anything odd about it, especially as Tuney seems to be routinely rather unpleasant to Lily. At any rate he responds to Lily's statement that she doesn't want to speak to him and her "look of deep dislike" with friendliness and a successful attempt to cheer her up.
That he seems hardly affected by her anger here may be a sign of insensitivity on his part, or of a tendency to fantasize her rather than really seeing her. Or it may just be that he is used to people being angry and cold with each other one moment and friends the next because that's how his family are - or because that's how Lily is.
The fact that Lily was already so cold towards him - that she automatically took Petunia's part against him in any dispute even when Petunia was clearly behaving badly, and looked at him with "deep dislike" rather than hurt or anger - suggests that Lily never was anything like as fond of Severus as he was of her. Yet, it was not a case of him pursuing her and forcing friendship on her: we see her apparently excited by his company and by what he is telling her, and she allows him to think of her as his best friend for five years at Hogwarts. That suggests that she was consciously or unconsciously using him as a source of information, rather than really loving him - or perhaps that she was naturally rather cold and her friendship was always conditional and easily lost, to everybody.
It looks as though Lily was never as warm to Severus as a good friend should be, although he believed them to be best friends. Yet, this was not fantasy on his part. Teenage boys are rarely very good at reading social cues, and he seems to be a lonely boy who probably doesn't have any other very close friendships against which to measure this one. If Lily reassures him that they are best friends - as we see that she does - why would he doubt her?
The scene in the courtyard is especially awkward. Superficially, Severus appears to be being a prat, and Lily certainly thinks that he is. But we must remember what Remus said about the werewolf "prank" - that Severus actually saw him in were form, transformed or transforming, and was bound to secrecy by Dumbledore.
When Lily accuses Severus's Slytherin friends of performing Dark Magic and he replies "What about the stuff Potter and his mates get up to?" he isn't just changing the subject, or trying to make two wrongs make a right. He knows that she is criticising Slytherins for a bit of spitefulness whilst Gryffindors are, quite literally, getting away with attempted murder. When she accuses his Slytherin friends of pulling a prank which is "evil", he knows that the Gryffindor boys' idea of a prank is to arrange to have somebody eaten alive by a monster, or infected with a disease which will ruin their life.
When Severus says he "won't let" Lily associate with the Marauders, he probably isn't being jealous or controlling: he knows that they are seriously dangerous, even psychopathic, and that she doesn't know and he isn't allowed to warn her. If he is being a little bossy in his desire to protect her, he is certainly being no more so than Harry is when he tries to order Ginny to sit out the battle in DH.
And, of course, he is right to be concerned. We later see James threaten to hex Lily, and even after they start dating James deceives her in order to go on hexing Severus behind her back. If they had lived, he would probably have deceived her in other ways too. Sirius and Remus proved to be less of a threat to her than Severus had feared: but Peter killed her.
Again, in the courtyard scene, Lily treats Severus rather badly. It sounds as if she has reason to be concerned about Avery and Mulciber, but she prefers to take the Marauders' word about the werewolf incident over Severus's and just assumes that he is "being ungrateful", and doesn't give him the slightest credence even though they are meant to be years'-long friends. And he repays her, as ever, with protectiveness and stammering adoration.
It also seems clear from the way she speaks ("I heard what happened the other night") that this is the first time Lily has discussed the Shrieking Shack incident with Severus, even though it happened at least a day and a half ago ("the other night", not "last night"), and she knows that he was in sufficient danger to require to be "saved". So, she knows her friend has had a horrible traumatic experience, she knows he's been in severe danger, and yet this is the first time she's spoken to him since it happened - she didn't seek him out to see if he was all right.
And now that she is speaking to him, for the first time since he was nearly killed, far from centring the conversation around his danger and his trauma she evidently sought him out to criticise him, since the scene opens with him saying "I thought we were supposed to be friends? Best friends?", which means she's said or done something which makes him doubt her friendship towards him. Contrast this with the way in which he will later throw away his political allegiance, his entire future and eventually his life because he thinks that Lily is in danger, even though they have long ceased to be "best friends", or friends at all.
In fact we see repeatedly, in their childhood interactions, in the courtyard scene and during and after the bullying incident, that even when Lily protests her friendship with Severus and makes at least some effort to protect him, she seems to have no care for, or no awareness off, any emotional suffering he may be going through. Either she is fond of him in a possessive way, as an item of her property, but has no care for him as a person; or she does care but she is somebody with a singular lack of empathy and emotional imagination.
In the bullying scene, Lily was slow to intervene on Severus's behalf. When she did, she didn't actually go to his aid even though he was choking - although that may have been because she was wary of being hexed by James if she took her eyes off him. According to JKR she already fancied James and was actually flirting a bit with him in that scene, over the top of her supposed best friend's distress and humiliation and despite the fact that James actually threatened her with violence. She does not even consider an offer which might have bought the Marauders off her friend's back for life (James said "go out with me" and he would spare Severus - he didn't say "go out with me more than once"), and she smirks when she sees Severus hung upside down.
In a brief moment of rage - and perhaps to protect himself from the consequences of being seen to be rescued by a Gryffindor girl, and perhaps to protect her from being hexed by James - Severus insults Lily and rejects her help in an offensive comment made to James, not to her. Lily's retaliation is ferocious, joining in in calling him "Snivellus" and jeering at his shabby clothes, which she knows very well that he is sensitive about and cannot help. Then she says that she doesn't want James to make Severus apologize - implying that she would accept a voluntary apology - and leaves him to his fate.
So Severus has suffered this horrible, public humiliation, including what is basically a minor form of sexual assault (whether or not they really did strip him, although they probably did), yet all he can think about is putting things right with Lily. She has already taken her revenge on him by attacking and jeering at him in public, and she has indicated publicly that she wants his apology which she is now rejecting, yet he has no anger in the face of what we are told is her pitiless contempt: only meekness and apology and a willingness to humiliate himself further by making a public spectacle of himself in front of all the Gryffindors.
It seems clear from the way Lily speaks to Severus - "None of my friends can understand why I even talk to you" - "my friends", not "my other friends" - that Lily had already rejected him in her heart months if not years before, and was just looking for an excuse to get shot of him. And when she rejects him, and when she goes out with the bully who tormented him and still torments him, he still isn't angry with her. According to JKR, one of the main reasons he joined the Death Eaters was because he thought it might impress Lily.
[This doesn't seem very sensible, even for a teenage boy. It seems more likely that if his decision to join was in some way serving Lily, it was because he thought he could protect her better from the inside. Either that or his friends had misled him, he really, truly didn't know how racist the Death Eaters were - JK has said, after all, that they actually tried to recruit Muggle-born Lily, so why would Severus think they hated all Muggle-borns? - and he thought that Lily only fancied James because he was "cool", and if he was cool too she might fancy him instead. After all, he knows that Lily fancies James even though James threatened her with violence ('Ah, Evans, don’t make me hex you'), and from Severus's perspective, at least, James is a thug who has nothing to offer except his popularity. It's not unreasonable for him to think that Lily is prepared to put up with a lover who is a potential threat to her so long as he is cool enough, or is even turned on by threat. It may well be that the dynamic between Severus's parents is like that - that his mother is in love with a bully who ill-treats her - so all in all if he really did think Lily might be impressed by him becoming a Death Eater, that's not as outlandish or as daft as it first appears.]
Lily tells him that he has made his choice, that he has chosen his creepy Death Eaterish friends over her: but in what we see, she never actually offers him a choice. She may have thought that the offer of a choice was implicit but teenage boys tend to be slow on the uptake, and need to have these things spelled out to them. At no point that we see did Lily ever say "Choose between them or me", and if she had done, he would probably have chosen her. But her behaviour suggests that she didn't really want him to choose her. He wasn't cool enough to hang out with her Gryffindor friends and, to be fair, the realisation that his feelings for her had become sexually-charged whilst hers for him had not probably made her very uncomfortable.
It's true there's something slightly obsessive about his unswerving devotion, even when she repeatedly kicks him in the teeth, but he certainly isn't a stalker in the usual sense of somebody who imagines a relationship where none exists, and then turns nasty when their imaginary affection isn't returned. Lily encouraged him to believe in their friendship, and it is she who turns nasty: apart from the one three-second burst of desperation and verbal rage, he never does, that we see. Coming from the sort of family he seems to, he probably doesn't see her nastiness towards him as unusual or as any reason for not loving her, or any reason why they shouldn't get together. He expects that couples will fight, and that the people he loves and who ought to love him will be nasty to him.
And if he is more worshipper than friend, worship seems to be what she requires of him: since she rejects him utterly to punish him for one brief lapse from adoration.
When Severus finds that he has endangered Lily, even though she has rejected him, even though she has married his tormentor, he throws over his allegiance to Voldemort and risks everything to save her - putting himself in extreme danger from both sides, in an era when both sides are torturers and killers.
Dumbledore calls him disgusting because he only asked Tom to spare Lily, not the whole family, and because it's Lily he is most concerned about. But really, it's natural for a not-especially-mature lad of twenty to be more concerned about the fate of a woman he is in love with than about an enemy or an infant he has never seen. And despite Dumbledore's comments, he isn't sacrificing Harry and James when he asks Tom to spare Lily: they're doomed anyway, and there's no way he can save them, other than by doing as he has done and defecting. What excuse could he give Tom for wanting to spare a child whom Tom perceives as a threat, and a man Tom probably knows he has good reason to hate? If he tried to get Tom to spare all of them he would lose all of them, and himself - or at least Tom's trust in him - to no good end. Whereas he does have a chance of saving Lily, and Tom does in fact make a half-hearted attempt to honour his request.
If he had not, of course - if Snape hadn't asked for Lily's life, and Tom hadn't offered it to her - there would have been no willing sacrifice and no blood protection and no rebounded curse. Tom would simply have killed Harry, and taken control of wizarding Britain.
At no point in the conversation between Severus and Dumbledore does Severus actually agree that he had offered to trade Harry and James for Lily, or even say that he only cared about Lily. He seems tongue-tied and confused by Dumbledore's accusation, as he was by Lily's, and all he actually agrees to in Dumbledore's tirade is that he asked Voldemort to spare Lily.
And Dumbledore sacrificed any claim to the moral high ground when Severus asked him to save all the Potters, then, and Dumbledore replied "And what will you give me in return?" Severus held to his side of the bargain - that he would do anything - but Dumbledore was lax in his, since he borrowed the Cloak which "hides from Death" from James and failed to return it, even when he knew the Potters were in extreme danger. Severus did indeed, as Dumbledore said, put his trust in the wrong person.
[It is irrelevant whether the Cloak would have saved them, in the event. Dumbledore couldn't know in advance whether it would save them or not but he must have known there was a good chance that it might, yet he still hung on to it.]
When he learns of Lily's death Snape reacts with true grief, not just guilt: nobody howls in agony out of guilt, unless they are some kind of self-dramatising religious fanatic. Yet he does not rage at Dumbledore, who has failed to keep his promise, and who treats him with great cruelty, reminding him of Lily's eyes when he is in the jaws of his misery, when it would have been enough to say "There is still something which you can do for her." Just as he never blamed Lily for ill-treating him, so he never blames Dumbledore, until the point where he learns that Dumbledore has used him to set Harry up to die.
[Actually, when we see his interactions with Lily and with Dumbledore, Snape seems remarkably free from resentment. This suggests that his sniping at Sirius is not so much because he bears him a grudge but because of the ferocious competitiveness which landed him in Slytherin in the first place. He doesn't want to hurt Sirius: he just wants to prove he's won. And his tension about Harry is partly because the fact that Harry has Lily's eyes in James's face is a constant reminder that James bested him in the battle for Lily's affections.]
We know that Snape kept the second page of Lily's letter to Sirius, and left the first page, which Harry later found crumpled, but we are not actually shown whether Snape crumpled it deliberately. He could have done; or Kreacher could have done; or it could have been crumpled when Snape found it; or Snape could have done it accidentally by clutching it too hard. Psychologically, the most likely person to have crumpled it is probably Sirius, when he re-read Lily's comment about Wormy seeming a bit down, but if the letter has spent a year or more slotted into Bathilda's book - as it seems to have done - that would probably have flattened it again.
Either way, Snape seems callous or obsessive when he tears the photograph apart so he can take only Lily with him, and takes only the page of her letter to Sirius which has her signature on it. But how did the letter get to Grimmauld Place - since Sirius wasn't living there when Lily wrote it - and how did Snape know the letter even existed?
The most likely explanation of how the letter got to Grimmauld Place is that it was with Sirius's effects which were returned to his family on his arrest. Snape might surmise that: but unless he and Lily had actually become friends again before her death, or Sirius had taunted him with his friendship with her, he couldn't know for sure that Lily had written to Sirius, or that Sirius had kept the letter. It seems unlikely he would have taken the risk of searching Grimmauld Place for a letter from Lily just on spec - or that it would just happen to be a letter which pointed Harry down the road to Bathilda Bagshot.
We can assume, then, that Dumbledore knew the letter was there, and sent Snape to get it. We are never told why the Potters refused to have Dumbledore as their Secret Keeper, but considering this letter it seems likely that it was because they had found out about his connection with Grindelwald; and Sirius may well have admitted having such a letter when he argued with Dumbledore, either before or after his time in Azkaban. Albus's repeated undermining of Sirius, his cutting him out of any sort of decision-making loop to do with his own godson, may well have been because Sirius had something on him and he wanted to weaken his influence.
So, Albus sends Snape to Grimmauld Place to retrieve the letter, to conceal his connection with Grindelwald from Harry. Snape takes the part which refers to Grindelwald, but he leaves the rest: perhaps because Dumbledore has told him to, in order to steer Harry towards Bathilda. Bathilda would then be very likely to tell Harry about Grindelwald: but perhaps Albus does want Harry to know about that, in order to point him towards the Hallows, but he would rather Harry hears the full story from somebody who was fond of Gellert, rather than be shocked by a casual out-of-context revelation.
Or perhaps Albus wanted Snape to retrieve the whole letter, but he chose only to take the part dealing with Grindelwald, and left the rest - so Harry could see what Albus had done. Either way, it seems likely that the rage and grief which left Snape weeping and crumpling the letter in fury (if he did) was not only love of Lily, or resentment of Sirius or of the married happiness which Lily described, but the revelation that all the while Dumbledore had been supposed to be keeping the Potters safe, he had been hanging on to their Invisibility Cloak for himself, even when they were in the greatest danger. Perhaps, too, he was affected by the realization that it was Dumbledore's link with Grindelwald which must have led Lily to put her life in Peter's sweaty hands instead.
Tearing Lily out of a family photograph seems harsh: but really, it's completely natural that Snape would want a photograph of the girl he loved, and completely natural that he wouldn't want one of the bully who made his schooldays a living hell. And apart from his general dislike of Harry, Harry is a very sore point by now and a source of misery and guilt, because this is after he has found out that (as he thinks) he and Dumbledore have raised the boy for sacrifice. It's understandable that seeing a picture of Lily with her son would distress him.
Nor is the rough, rather destructive way he chucks Sirius's papers about evidence of any character-fault. Harry loved Sirius, but he throws his things about equally casually in an attempt to find more of Lily's letter.
One might ask why, if Snape loves Lily and protects her son for her sake, he isn't also ni
