OK. I’ve got a theory.
So, Grindelwald did not attend Hogwarts, but if he did, would he have been in Hufflepuff??
So, every house has their bad guys....
Gryffindor: Peter Pettigrew (+/- Marauders -m a y b e )
Ravenclaw: Gilderoy Lockhart, Quirinus Quirrel
Slytherin: Voldemort, Bellatrix Lestrange (+ etc.)
You know, except Hufflepuff. Also, Grindelwald was apparently loyal to Dumbledore. IDK really, just a thought.