Here’s something that gets me.

“There’s not a witch or wizard who went bad who wasn’t in Slytherin.”

Well okay, let’s see that hat that gives you a house can read souls or something. It can tell the kids who are cowardly, power-hungry, spiteful, bullying. And what does it do?

IT PUTS THEM ALL IN THE SAME HOUSE.

It’s a self-fulfilling prophecy. Not only does it put all these bad kids in the same house, it puts them in together, so that they get even worse. And it gives them a rather sour, favouritism-based teacher.

Rather that scattering them all carefully so that they can be influenced by the nice kids, it shoves them all in together.