But I don't think that you can distinguish Snape's cruelty as a teacher so easily from his cruelty as a Death Eater. He isn't just a guy who's sarcastic, antisocial, and hard-to-get-along-with. He's mean. He enjoys using his power to hurt others. These are not quirks of character that we should just accept and enjoy; they are problems: they make him into a person who is Not Good.

I realize that this makes it sound like I'm arguing, "Snape is mean, and therefore he is evil," which I'm not at all. But Snape is on the fence, or was -- he is partially attracted to evil. And so for him to come out good, he would have to repudiate some of the unpleasantness. Sarcasm is one thing. Not suffering fools is one thing. But tormenting his students? That to me indicates that he is still in Voldemort's thrall, even if he doesn't think he is.