Slytherins are always cast as villians...because they are

    I see a lot of "all slytherins are not all evil people! Stop vilifying the villains!". Maybe the reason people tend to cast them as villains is because they are the villains?
    One of the defining traits of a slytherin is cunning and ambition to the point of exploiting people. That is not a positive defining characteristic. Just saying. More than that aligning yourself with slytherin is saying you want to be aligning yourself with prejudice, violence and evil. It isn't that everyone in that house is pure evil (Slughorn and Regulus are literally the only arguments and let's face Regulus wasn't really a good person, he became a good person) but their desire to be grouped with people who are is at the very least a red flag and at most they're Voldemort.
      Even if someone valued slytherin qualities to an extent that doesn't require harming others (Slughorn) they would have to at the very least tolerate (but commonly desire) to be associated with mass murders and prejudice. The example of a real slytherin that wasn't placed in slytherin solely because of their adamant refusal to be aligned with hatred is Percy Weasley. He is a slytherin through and through, to the point of stepping on his family in his assent to success but ultimately he was unwilling to come anywhere near the dark arts and the evil people who studied them. It's really too bad more people in our country had the integrity and honesty to remember that support for specific people can not be divided from the racism, sexism and general evilness of certain people.