Humans have an incredible capacity to be selfish tits, who cruelly ignore the cruelty of others. It's all some petty emotional game.
I ran across this totally by random, but it's an excellent hammering home of the simple historical truth that the German Nazis committed mass murder of millions of people by using poison gas chambers.
Philosophically speaking, one of the few things more horrible than cruelty is pretending it never happened.
The old line "those who don't learn history are doomed to repeat it," holds true time and time again.
As a species, we need to accept the horrible shit we've done, so that we stop doing it over and over.
As a species, we need to accept the horrible shit we've done, so that we stop letting ourselves do it to us over and over. But, that's a drift into It's Class Warfare, Bitches.