This line of argument always seems weird to me. The Imperial Japanese Army was made up of conscripted _civilians_. Fathers, sons, brothers, uncles; ordinary folk from all walks of life were conscripted into the IJA and given training and sent off to invade other parts of Asia, where they committed various atrocities on a par with the Nazis. Civilians did do that.

As for why, IMO the strong militarism that was part of their culture in that era is probably a large part of it: https://en.wikipedia.org/wiki/Japanese_militarism