I think one of the reasons that sort of thing abounded in early American comics would simply be that we hadn't had the chance yet to see real atrocities in action. In the '30's, when most of the things got started, America was still very sheltered in a lot of ways. Sure, there was the Depression, which was no cakewalk, but compared to most of the rest of the world, which was shaken by war and colonial conflicts, we were shielded from most of the true horrors. We didn't have much to do with WW1, compared with, say, the Brits - largely, we only heard descriptions of all of the carnage and whatnot that the rest of the world was suffering, and to our innocent minds, it sounded kinda cool, in the way that war always does to those who have not been embroiled in it. So we put all kinds of freaky things in our comics, not knowing the reality of them, just putting them in there because they sounded good at the time, and through WW2, we were so swollen with anger against the enemy that virtually anything went, as long as it was directed against the Nazis. It was only afterwards, once the bloodlust had faded and we'd seen the true results of war - concentration camps, etc. - that we started to lose our taste for such things. Anyway, that's my take on it. May or may not be true, but it sounds right to me.