I think the outlook of the Wizarding World as given at the books is very biased. Even Dumbledore who is suppossedly all wise at the series is most judgamental of an 11-year-old and of the Slytherin House at all, since the 2nd book was such an anti-Slytherin propaganda
But I think Rowling might have done it on purpose, to make it very difficult for us to feel sympathy for the characters. (In a way, she failed, considering all Snape fans, heh.)
After the 6th book, which wasn't my favorite but oh well, the good thing about it was that Rowling for the first time was making "Slytherin" be seen from a new light. Draco Malfoy, who was in Harry's definition the worst boy he ever met even worst than Dudley, is finally seen as a victim and as a not-so-bad person. Even Harry now wants to help him.
On the other hand, I don't believe in good and evil so that has it. If our side is always good and the other side always bad, then there's no real definition of good and evil and really... asking from Harry's pov if the Slytherins are evil? Then yes. Ask from the Slytherins' pov who's evil? They will say, the Gryffindors (or even the whole school).