Why did the nazi and Japanese treat prisoners and others so inhumanly?

Their belief systems were breeded into them from the time they were young. Brutality and cruelty was nothing new to the Germans. They did the same thing in World War 1. The Japanese had it taught to them from the time they were young too. The Japanese have a strong honor code (which does not have our ideas of honor) they followed. Their code compelled them to treat people badly. Trust me, the USA did do their own method of re-training their ideals, values and honor systems.