Yeah, the Italians were pretty brutal domestically and in north Africa, and were the original fascists, but I'd say the Japanese were probably worse. As a non-historian, it seems complicated by the fact they were a copy of the European empires they were contemporary with, who also did messed up things in colonies, even if according to the historians I've read they did go further.

The Nazis were very open about wanting all the terrible things, and very distinct from the people they fought against, so they go in the worst-of-the-worst bucket with ISIS and Dixie.

And as horrific [as] chattel slavery has come out to be

Was ever fully forgotten? Like, it's definitely spoken about more openly now than in the early or mid 20th century, but the abolitionists of the day and living memory afterwards were quite active documenting exactly how fucked up it was.