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 is still murder. If mass murder is a crime or sin, so is the murder of an individual, however depraved.
I’ve never heard of a story where someone decides to simply cripple the political career of these individuals. Say, being at the right place at the right time and making sure Hitler ends up as a quadriplegic, or even just an amputee short one leg, but with a voice that cannot be used for oration. Mao, blinded and morbidly obese, would not have become the Chairman we remember. Pol Pot, had he ended up locked away in a loony bin or kidnapped in Thailand or shipped off to France, would never have managed to lead Cambodia into its insanity.
Of course, the question remains whether we’d have seen the same kinds of horrors despite such intervention, and again, this is a question SF has dealt with.
What I haven’t seen dealt with, yet, is the bigger question of what it is in human nature that makes us so susceptible, prone — indeed, in the aggregate, eager — to be led into insanity by complete assholes. And what, equipped with a time machine, someone — moral relativist or absolutist alike — might decide ought to be done about that.