When I think of Snape being such a *insert bad word here* I automatically think of James, Sirius and the others mocking him. He definitely made his mistakes like becoming a Death Eater and all but when I think about what lead him to his mistakes and that he still was a good guy he is no good "evil people in Slytherin" example, if you ask me. He is one of those characters like Harry and Dumbledore which are shown to have their dark sides or troublesome past and who still are the greatest heroes in their world. I mean - even Draco was just some coward who did what he did because he grew up in an intolerant family and he just did not want to get killed by Voldemort.