I am no military historian, but it's my understanding that many armed conflicts that we might consider pre-civilized concluded with just this kind of slaughter (and pillage, enslavement, etc.), and that the widespread recognition of civilized rules of war has saved literally countless lives. As bad as the Nazis were, I think it's unequivocally a good thing that we were not forced to depopulate Germany. The reason we weren't was that Germany surrendered, and the reason Germany surrendered was its well-placed faith that we wouldn't depopulate (or torture, enslave, etc.) the nation anyway.
Leave aside everything else: How do you use the phrase "literally countless" without giving yourself whiplash.