Americans, and Westerners in general, are so used to the idea that all people have worth that we fail to understand that most of the world doesn't think so. I include in that list not only most people across great stretches of time, but fairly near ancestors of ours as well. It is not at all natural to human beings to think that way - not at all accidental that most tribes named themselves "The People" or "The Real People." We find that almost quaint with native tribes, but that is also the meaning of "Das Volk," and names of some European political parties.
We think that to be willing to kill many people in another tribe must require not only rage, but some unaccountable ability to sustain anger over months or years. Not at all. The default position is the contempt of not regarding them as human, punctuated with periods of anger when we need a burst of energy. In our history, we can see it best with slavery - do we really think that generations of white people woke up every morning with a seething anger at blacks? Of course not. The attitude was generally to disregard them entirely except as pieces of the environment. They might even have some affection for many of the individuals. But regarding them as less than human was the key.
So also with the amazing physical cruelties of invading Mongols, North Sudanese, Indonesians in East Timor, Mayan overlords, Romanian voivodes. Those others were not real people. What does it matter if they felt pain? All that matters is protecting my people.
Well, why should they care? Run the exercise in your mind - not of the people we encounter in our day, but of any random tribe you might think of on the face of the earth a hundred, a thousand, or ten thousand years ago. What would be their basis for having the least concern for other tribes, except perhaps as trading partners or useful allies. Why not burn their crops and steal their women?
How did we ever learn differently?