I think in the context of world war 2, the nuclear bombs were just a more powerful instrument of destruction. If anything, they saved humanity from more devastating total war with conventional arms, which would have killed millions more.

With the advent of the B29, the US was dropping napalm and phosphorus on Japanese cities to create massive firestorms. The glow of Tokyo burning was visible for hundreds of miles. On the scale of horror or inhumanity, I’m not sure how to compare different types of aerial bombardment targeted civilians as instruments of war production that needed to be eliminated.