“Killing an enemy with teeth and bare hands is emotionally far more demanding than pulling a trigger or pressing a button. In inventing tools and weapons, in contriving civilization, we have disinhibited the controls - sometimes thoughtlessly and inadvertently, but sometimes with cool premeditation. If the beasts who are our nearest relatives engaged recklessly in … mass murder they would have rendered themselves extinct. If our non-human ancestors did it, we would not be here.”