Quite.Slytherin=evil (not the prejudice, the fact that it's true) is my least favourite thing about the books, probably. It's childish and obvious. A relic of the more childish book 1.
Dumbledore: It is our choices that define us, Harry. Only we can decide if we use our gifts for good or ill. Oh, and Voldemort just happens to be the sole remaining descendant of Slytherin. Coincidence, I'm sure! By the way, is Remus a fox, or what?
Harry: What?
Dumbledore: Never mind.