I think that Snape was a good person, he was just led down the wrong path. it really makes you wonder the kind of things his mother would have given him to read when he was a child that could make someone so greatly want to be part of a dark society, such as voldemort's. I believe the main thing to remember with Snape, is that he probably thought he would impress Lily by becoming a fully blown death eater and that losing her practically destroyed him. he is unncessarily cruel to the students, but i find it interested that because he has been void of affection for so long that he doesn't himself know how to give it.