Or when people say "You can't be in Slytherin, you're not evil." You don't understand Slytherin House at all if you think that's what Slytherin is about. << Or when they say "But Slytherin is the evil house.
It is just that Slytherin has a longer list of dark wizards. Gee, I wonder why. Maybe they are treated like trash by the other three houses and want revenge. Who would have thought that Slytherins have feelings as well?