Well, the sorting hat keeps harping on the notion that the four houses need to pull together. I think that Slytherin house is so full of a-holes lately because Snape fosters it. Riddle and the Death Eaters not withstanding, Slytherins were probably tolerable back in Slughorn's days.

Something else bothers me ... what house was Pettigrew in? They make it sound like he was a fellow Gryffindor, but that contradicts the statement "There's never been a witch or wizard who went bad that wasn't in Slytherin." Was Quirrell a Slytherin? Was Lockhart? Percy certainly wasn't. I think the statement must be overreaching.