Think about it - nowadays we go on about how polite and civilized the Japanese are. Well, it wasn't that long ago that they were a nation full of totally indoctrinated people, ready to kill and die for their country and visit almost any cruelty on anyone else.
Biologically and what has really changed? Was there some pathological madness around in the 1930s and 40s? It can only be a social thing. People in groups or individuals, who weren't inherently bad, can be persuaded to do bad stuff. It's a real big stretch to assume that everyone in Japan had a bad upbringing, or was mentally ill.
Religion of peace, obviously.