I'd like to point out that saying there's a pretty big mistake.
"The best that can be said is that while every dangerous lunatic wizard was a Slytherin, not all Slytherins were dangerous lunatic wizards."
I can point to Peter Pettigrew to debunk that. Not only that, there is no demographic information given about which houses turn out the most dangerous lunatics (I'd vote Gryffindor myself. Ambition lends itself to not being a lunatic, bravery and lunacy however are just two sides of the same coin...) The only bit of information we have is the word of Ron Weasley. Sorry, Ron really REALLY isn't a reliable source of information.