Snape is a mean, ugly shnook.

[prefectmarcus]

No: HPFGUIDX 38137

Snape grew up with a lot of hate.  He drifted, as so many hate-filled 
people do, into a hate group.  At first it was great.  A little 
torture, a little imperious curse, a little vandalism was fun.  Then 
the murders started.  He was horrified, but by now he was in over his 
head.  Dumbledore offered him a lifeline out.  At great personal 
risk, he managed to escape.  Dumbledore gave him a job, trust, and  --
 most important -- his self-respect back.  There is simply no way he 
is going to louse that up by going back to a psycho!

He is still a mean, ugly shnook, however.  He is not nice.  He is not 
pretending.  He doesn't do it to prod people into doing better.  He 
does it because he is mean, spiteful, and hateful.  To quote 
Quirrell, "Oh, he does (hate you), but he never wanted you *dead*!"

Marcus