Un Title

I’m rereading Harry Potter because it’s summertime, they’re familiar, and I’m out of new Meg Cabot books. And I stumbled across this statement, said by Hagrid & Ron in the first book:
There’s not a single witch or wizard who went bad who wasn’t in Slytherin.

Does this mean that every evil witch or wizard has been in Slytherin?
Or does it mean that every witch or wizard in Slytherin has been evil?