Personally, I don't think he's nasty for the fun of it. I don't know why, though.
I love his sarcastic sense of humor. Especially in (I think) Prisoner of Azkaban, when Hermione's teeth are growing, and he looks at her and says, "I see no difference."
My jaw dropped when I read that. I was laughing, though. It's sooooo bad it's funny. Cruel, but all the same....I'm still chuckling about it.
I love Snape. The series just wouldn't be the same without a sarcastic jerk like him.