Humans are aggressively territorial creatures. As resources dwindle in one area, the tribes of humans will spread out, looking for better territory elsewhere. They will kill those who are weaker than they are, and eat their food and take over their lands. Have you ever watched Meerkat Manor? It is really pretty deep. You might cry out, "But you are all adorable Meerkats! Why must you battle and slaughter one another and eat the babies of the losers?" Humans are very much the same. Some may have more or less mercy, may merge tribes instead of killing them, or adopt the babies instead of eating them. But not much more often than Meerkats do. Humans  commit depravities, because they are so smart that they can kill each other in more inventive ways than the other animals. They never seem to overcome their basic territorial and brutal natures. We can decry history, and the hunger of our forbears, but does a wolf feel guilty for eating the lamb? Can a wolf evolve into a gentle sheepdog?