Actually, your hypothesis is sound. Social psychologists have long known that humans mentally and socially create in-groups and out-groups. As a result, prejudice arises. And when two groups are competing for ... land, resources, ideology, political power, or whatever... That prejudice becomes something far worse, as observed in Sherif's Robbers Cave Experiment.

As one very influential social psychologist once said, "When the chips are down, these... these civilized people, they'll eat each other."

Sad, but true. Civilized society allows for complex thoughts and reasoning, as well as our complex moral codes. This is because we have time to think about such things. We are not in immediate danger. But when society collapses, or when we're in a situation that calls for an immediate response, the lizard brain will take over.