I remember seeing plans for a crematorium. The drafter bragged at how efficient it was as it channeled melted fat from the corpses back into the furnace, saving fuel.
I read that and wondered how anyone brought up in the West could calmly sit down and design something like that, let alone brag of its efficiency.
Had I run into that person, I would have had no qualms in cold bloodedly executing him.
Nazi Germany, The Soviet Union, Communist China, the Khmer Rouge, Islamic Iran, and ISIS all have had little trouble in finding people to directly or indirectly use violence and mass killing to gain and maintain power. Nazi Germany was simply more efficient: killing at a mass scale, against specific populations, in a comparatively short period of time.
Barbarism does not live very far below the surface of civilization.