I seriously doubt that EVERY SINGLE WIZARD who went bad came from Slytherin. What about Wormtail? He was in Gryffindor but he went bad!

I think Wormtail was more scared then evil. How can you explain Draco Malfoy getting into Slytherin? In the words of Dumbledore "You know sometimes i think we sort to soon.."

I don't know....I saw Wormtail and more power mongering - or so insecure that he had to be with whomever was going to be the strongest (or who he thought was going to be the strongest)

And the same can be said for Regulus Black.

It's more that the traits that Slytherin's posses are often also pocessed by those we term to be 'evil'. But it's all about how you use those traits

Well, it come's to my mind that the sorting hat kind of can tell what kind of person you are. That is not to say people can not change. The other houses all despise Slytherin as they think they are above everyone. I think that is one reason we hate Slytherin, as our heroes do. I doubt every single person that went to the dark side, were in Slytherin.