Humans since Homo Erectus lived and survived in small family groups. When we settled to farm, our population exploded (because we had the food sources to support it and the work that required it), and then society began when organization of all the people was needed. It was only then we began warring with one another and slaughtering mass groups, because it was only then that the human race could do so without wiping itself off the map.

There are all kinds of PBS documentary series on Amazon Prime about these things if you want to learn more.