It really annoys me how much you see Slytherin bad Gryffindor good. Every single Slytherin is characterized as a bad person just for their house. It's emphasized that the most dark witches and wizards come from Slytherin. Even some of the characteristics of Slytherin are 'evil'. Cunning and self preservation aren't characteristics a hero has.
It's forgotten though, some of the good traits of Slytherin. Ambition, pride, determination, resourcefulness. It's not all Muggle-hatred and evil. Slytherin himself was obvious a pure blood supremacist, but all of those in his house don't have to be. There's also the distinct lack of good Slytherin witches and wizards. Phineas Nigellus was the only Slytherin Headmaster to have been fairly appointed, Snape is the closest they're is really to a good Slytherin. You'd have thought there would be more, as ambition would lead you to high places like Hogwarts headmaster and people do have ambitions to do good. For a series that has a preaching of acceptance for people beyond things they can't control, like their blood, there is a lot of prejudice against people that are in Slytherin