I also think Slytherins were given a biased reputation. I have to agree with the fact that most of this was from Harry's point of view, which was partially influenced by his parent's death and Hagrid's judgement, so naturally, he hated Slytherin. Immediately, we get a biased representation of Slytherin.

But I also think the people in Slytherin choose to be in Slytherin because of their family. Maybe their character traits are different, but their choices are Slytherin so they end up there. I remember Dumbledore saying something about our choices showing who we truly are far more than our abilities, and I think that may have led to the students with a history of dark arts being in Slytherin. The are influenced by their families who supported Voldemort, so, consequently, Slytherin ended up having more Voldemort supporters, who hated muggle borns and thought pure bloods were above everyone else.

I guess you start to see in the later books that some Slytherins like Draco were willing to work hard to bring family pride, a Hufflepuff or Gryffindor trait. Snape was loyal, which I believe is a Hufflepuff trait. But you don't see that until the end, and Harry's point of view sort of puts that in a biased way, even if you grow to accept them.

I'm not sure if I could understand that, but I think Slytherin was given a biased representation at the beginning, but the major characters (Draco) lived up to it. In the end, we see more depth though.