The whole point of the Conventions was like an early form of M.A.D. - which was a sort of unofficial convention in itself. The US broke the 'conventions' when they dropped the bombs, but once could argue that it was partly due to ignorance of the after effects.

Of course, the Japanese had a different culture, in that no true warrior would allow himself to be captured alive, but would fight to the death, so they had no respect for enemy soldiers who though any different. The European practice grew out of centuries of gradually more bloody warfare, and perhaps a bit to out of the medieval to 16C practice of 'war by mercenary', especially in the Italian states.

The point about Hitler was just how it was supposed to work. He was also gassed in WW I, so knew a bit about that too, and just how nasty it was...

The old saying about 'treat your enemies how you would like them to treat you' was partly born out of fear...