It is a fact that humans destroy the lives of other humans - strangers, friends, lovers, and kin - and have been doing so for a long time. These cases are unsurprising and easily explained: We harm others when it benefits us directly, fighting to win resources or wipe out the competition. In this sense we are no different from any other social animal. The mystery is why seemingly normal people torture, mutilate, and kill others for the fun of it - or for no apparent benefit at all. Why did we, alone among the social animals, develop an appetite for gratuitous cruelty? This is the core problem...