Am I the only one who finds the fact that a pre-requisite to being evil, is to be a Slytherin anti-climatic?

Throughout the series it is made clear that whilst not all Slytherins are evil, all evil people who have passed through Hogwarts have been Slytherin. Am I the only one who finds this anti-climatic? Yes, people are picked because of what they value. But has their ever been a witch or wizard who has gone from good to bad and not been a Slytherin. Human character is too intricate and complicated and you can't just put all the bad apples in one basket and have a blanket idea of what makes a person bad.