For years I have taken the "lesson" of WWII to be: look what HUMANS can do to each other. I have certainly spoke to many who took as the lesson: look what GERMANS can do to people.
After WWII, among other things, the allies forcibly deported (or expelled) more than 12 million ethnic Germans from Czechoslovakia, Hungary, and "Poland," where I put Poland in quotes because the area from which Germans were expelled included hundreds of square miles of land that had been Germany until the Allies gave it to Poland. The vast majority of the expelled were women, children, and old people. Expulsion was NOT tied to any concept of individual guilt or collaboration with the German enemy by those expelled, it was driven by the ethnic identification of the expelled wtih the German enemy.
Every kind of atrocity you read about in any massive expulsion happened in this one, the details are here. I won't repeat them in this post, but will mention two details. One is that about half a million of the expelled died in the expulsion of starvation, freezing, and presumably other causes. The other is that the Allies enslaved many of the ethnic Germans in labor camps including Theresienstadt and Auschwitz I which the allies took over from their former Nazi bosses. The slave labor program was referred to as "Reparations In Kind."
The lesson for me is that, at least at the ethnic or national level, there is no "they" there is only "we" when it comes to inhumanity. What I mean is, it is not "them" where "they" are the Nazis, the Stalinists, the Maoists, the Khmer Rouge, the European invaders of the Americas, or the Moslems who are the problem. The problem is Humans. We collect into large groups with highly specialized and stratified political, legal, and economic roles, and among the responses we typically turn to are 1) identifying a "they" that "we" can blame stuff on, oppress, and exploit, 2) lumping in to our enemy a broad range of people who by a slightly different analysis had very little to do with our enemy.
I don't propose or support self-flagellation over this woeful state. It is just another thing which is true about humans. Live with it. Be aware of it as a human bias, and say everything your neocortex can come up with to counter it.
In my opinion, being aware the problem is us and not them will lead to consistently better results in mitigating and battling the bias.