I have often wondered if something like this could ever happen again, but it already has. The number of aborted babies is the evidence. As far as this happening against adults & children as it did in Germany, well I suppose that might depend on when Christ calls his Church home as in the Rapture & what will accompany that in the resulting time of Tribulation on the earth.
This is one reason why I have struggled to understand it.
I lived among the German People only 25 years after the liberation of Dachau, Auschwitz, et al. Witnessing their humanity, their brotherhood with all the other people of the world, only baffled me even more.
So far I have concluded that the human mind cannot comprehend it; at least, mine cannot.
I have also sworn an oath never to be a part of anything like that, even in a small way, regardless of the consequences to me.