So you're either liars or idiots

Professor Snape and a few other slytherins were half-bloods who survived in the pureblood, muggle hating slytherin house. Even joined Voldemort's cause. What if there's a reason why they do that? More so, what if a muggleborn enters slytherin and proves that, just as blind as the other houses are of this truth about slytherin, slytherin is blind to the truth about Potter? HP AU