The best way I’ve heard it described is that they both view the other group of people as existential evil. Far beyond enemies, something which is evil just for existing. Not just the militaries, but the nation, race, state, religion, whatever classification. With that viewpoint, any action you take can be justified. Just as nobody would think twice about killing a million mosquito larvae in a country that has thousands die from malaria, killing a few thousand of the other side is morally neutral at worst.
This is going to continue to be horrific for a while.