Warded Ms. MacDonald/Mr. Malfoy.
I'm sure that it's what Dumbledore wants his Gryffindors to think, but unfortunately his dislike for the Slytherin house is well known and often shown, creating a resentment that pushes young people - young Slytherins - toward the opposite ideology simply to escape the unequal treatment received in school.
He doesn't simply fail in making students unlearn those lessons; he's guilty of ensuring that every Slytherin takes those lessons to heart and hates everyone that Dumbledore likes/approves of. I do wonder how much of what we've witnessed is really the result of the profound hatred Slytherins build up during those seven years in school.