Here’s a thought for those writing SF novels about time travel: The hackneyed plot is always one of those ethically dubious stories about someone deciding to get into the time machine and kill Hitler, or Pol Pot, or Mao, or whoever. They do so because they are supposedly moral relativists and believe that killing one psychopath is better than letting that psychopath lead a society into the mass murder sanctioned by governments led by psychopaths like Pol Pot, Mao, and Hitler. The moral absolutist, however — say, the traditional Catholic — would argue that killing Hitler, Mao, or Pol Pot …