I think our moral viewpoint has changed over time !

When we were in tribes killing animals, that was deemed necessary at that point of time, because of all the benefits of animal food and the shelter provided by the animal skin.

Then we moved into killing other humans, to gain control of their resources, because it was either them or us. I think it was correct from their viewpoint what they did.

It's unfair to judge them using our morality, maybe our descendants will judge us in the future for wasting our natural resources, who knows ?!