I don't have an insanejournal account, but I'm mezzanine on livejournal :)
Anyway, I haven't finished the story yet, but I'm astounded at how Snape perfectly fits the qualifications for Narcissistic Personality Disorder. I might just have it on the brain since my mom and I have been researching it lately, but it's an odd coincidence! He confabulates, he's prone to stalking and possessiveness, he twists all of Lily's words and actions so that he can excuse his bad behavior by pointing out what she has MADE him do, he believes he deserves to have everything he wants, normal societal rules do not apply to him (stalking, extortion, attempted murder), he uses circular reasoning, he flies into narcissistic rages when faced with real or imaginary slights, and most importantly he lacks empathy- he never thinks of how others feel. His family life is perfect for this too! His father seems to have NPD (especially since he has the coup de grace: grandiosity - convinced that he's a great potioneer even though his skills are mediocre at best), and Snape is constantly made aware of how lacking he is.
If you weren't aware of the similarities, you did a pretty bang-up job making him as nasty as possible! Narcissists are considered by most professionals to be untreatable, and the prevalent advice for family and friends is to get away :)