They organize everyone by "type" and keep them mostly separated throughout their tenure. Sure, every fan of the books has rushed to find out what house they are, but the sad truth is that the lines dividing these houses are largely arbitrary. Every person has qualities of a Gryffindor, Slytherin, Ravenclaw, or Hufflepuff. Children are hardly fully formed individuals.

But it's not like there's one house that is known for producing evil wizards. Oh, wait, there is a house that harbors people profiled to possibly be evil into a secret room together.

So obviously most of the Slytherins changed to the Dark way because they were made to think at a young age that all Slytherins were evil.

This is almost like controlling a child future and turning them evil.