I'm curious what the next act for leftist ideology is, since various forms of it (mostly communism) were responsible for the most human misery in all of human history in just one century.
I hate participating in these discussions though, because everyone’s characterizations of their opponent are so absurd, it’s like—where do I even start with you people?
I’d tell you the same I’d tell crazy liberals... go talk to some actual people and find out what they really think rather than basing your opinion of half the population on some crazy person you saw on the internet once.
And then there's the institution of slavery, which wasn't political until abolition efforts were underway. (Semi-related: hard to label abolitionists conservatives or rightists.)
It also bears mentioning that Hitler was anti-immigrant and nativist, which are the forces that animate today's Western conservative backlashes (including Trumpism).
That said, a little googling seems to indicate that the Mongols do, in fact, win that competition. They were pretty big on genocide.