I don't think we really learned any lessons. Stalin and Mao were at least as bad as Hitler in terms of body count. Hitler just lost the war, while the other two were able to continue to invest in propaganda to whitewash their histories. If Nazi Germany had emerged from the war intact, they too would have been able to whitewash their history for decades after. I'm sure in time they would have "moderated" a bit, and that little episode would be a regrettable footnote the way the Cultural Revolution is for the Chinese or Stalin's purges were/are for the USSR/Russia.

We have also never fully acknowledged what the Spanish, British, and later USA did to the native populations of North and South America in the preceding centuries. Yes it's true that disease did much of the work without anyone lifting a finger, but that's only true in the early days. Later on there was plenty of intentional slaughter including bounties to encourage it.

When I look at the way Twitter erupts into ideological witch hunts or things like the Qanon cult, I realize that we have not changed. If any of those mobs got actual power we'd see the same things repeated against whatever groups (real or imaginary) they hate. Civilization is a thin veneer.