The Problem with Inner Nobility
More random rants...
One of the enduring character traits than fanfiction versions of Severus Snape seem to adopt is an inner, hidden nobility. The idea is that Snape is really a noble, kind, compassionate soul who's forced into situations where he is obliged to act mean, nasty, or - in the case of Albus Dumbledore - downright murderous.
Personally, I can't stand that idea. It's something that will, almost invariably, stop reading a story.
The trouble is that it's really just a way to fundamentally change Snape's character without any character development. The author is simply saying: "Oh, so, y'know how Snape comes across as? Well, that's not the real Snape - all he needs is some love/kindness/understanding and he'll blossom into this super-nice heroic character that doesn't at all resemble his adopted nasty persona!"
See, I like reading about Snape. I like it because he's an evil character working for good. I generally dislike good characters; but I also generally dislike evil causes, so an evil character pursuing a good end is compelling. "Good" heroic characters tend to be interfering do-gooders, fixer-uppers, who stick their noses into things unasked and make a mess of things (all in the name of the greater good, or altruism, or something). "Evil" characters don't worry about pesky morality so much, and try to be as effective as possible. The drama that results is, also, usually not as nauseating.
Changing his character, fundamentally, means I'm no longer reading a story about Severus Snape. I'm reading a story about some poor sod who was forced into acting evil - first by his Death Eater friends, then by Dumbledore - and has been completely screwed over in life (betrayed friends on both sides, forced to act against his personality, etc). He becomes a character to be "rescued" from his situation by (frequently) some suitably perceptive attractive, nubile girl.
"Evil" characters don't need to be rescued, at least emotionally. They can exact revenge, selfishly move to a different situation, etc. The character is more "active" in that they are self-determining - they are in control of their own reality, or take control of it.
Furthermore, what makes characters distinctive are their flaws (and virtues). Having a soft goey center in the middle of a an outwardly nasty personality is a way of eliminating the character's flaws. The story becomes a "reconciliation" story, where the "rescuer" brings the "real personality" out into the open, and forces a reconcliation between their friends and the "real personality." It is not a story about the character; about how their flaws (and virtues) drive the character, cause them to succeed or fail, etc.
There are also at least one logical difficulty. If Snape is constrained by his situation to be evil, then how did he get into that situation to begin with? It does not seem credulous that he would have to act nasty to students and Order members to fool Voldemort or make him "seem loyal."
On a side note, one could argue that he would need to act nasty to Order members because he isn't loyal - a spy that was friendly with Order members would be capable of gathering more information, influencing people action's more, etc. A spy ostracized by those he's spying on would be expected to be useless, so he wouldn't have to divulge much useful information to Voldemort. This view may be supported by his behavior at the beginnig of HBP, where he is quite civilized when interacting with Naricssa and Bellatrix; then again, he might just like them more. This could possibly carry over to the students if he trying to deliberately alienate Slytheryns from the rest of Hogwarts, ensuring that Harry Potter isn't like to be betrayed by a false Slytheryn friend.
But all that is terribly convoluted, and unless you're writing a spy/thriller fic it doesn't seem worth it.
Snape's vitriol and hatred is a defense mechanism of some kind. But I would argue that making it a manipulation - by his goey center - causes it to lose its power. I would prefer to see it as reaction against (i) feeling trapped (by a boring teaching job, by saving a James Potter lookalike, by betraying his friends, etc) and (ii) a tendency to blame other people for his own losses, and likely failings.
Not that he's necessarily unjustified - it was really quite appalling for Dumbledore to allow the Potters to be killed, that is, using an Order member as a Secret Keeper when he knew there was a spy (and probably one close to the Potters, since Remus Lupin was a suspect).
In any event: it Severus Snape really does have a soft goey center, then how did he end up in the situation he's in? How could someone like that join the Death Eaters, and the only change sides when Lily Potter (a childhood friend/romance figure) was threatened? And then how could they be consistently cruel to children? That kind of cruelty takes a certain creativity.
Bah. I'm over-complicating it. I just don't enjoy it because it's a way of neutering the character, and starting from scratch. It's like, oh, having a character enjoy an epiphany after a very traumatic event (e.g. Hermione Granger being gang-raped. WTF?). You're wiping the slate clean, and then "writing-in" a new character. It's sort of like a self-insert, I guess, or a fantasy-figure insert.
Worst of all, it isn't really necessary. If you don't want to remain true to how a character is portrayed in canon, you don't need to - you can just write them as you like them. But by adopting a "reconcile true character" gimmick, or a "destruction/reconstruction" one, you're forcing a very specific genre (Hurt/Comfort), and frequently a very specific plot structure (e.g. Key Event/Reconcliation/Backsliding/True Connection/Ending). As mentioned, it becomes a reactive story, and not a character-driven one.