Everyone ridicules you when you are a Slytherin. They blatantly disregard you or whom you might have been. With all those turned backs, you feel yourself slipping, indulging in those of your house. Maybe you haven't always been that way-but Merlin knows nobody's trying to stop the change. You are a dark wizard...right? Being a Slytherin and all. You hate muggles and all you consider 'lower than yourself'...don't you?
Maybe Slytherins are bad wizard's-7 years of being hated by ¾ the school's population can really change a person. But who's to say that when they were younger, they weren't innocent to the prejudices of the world? Or that they heard anything but what came from their family, and with that you can hardly blame them for how they are.