If you want to go back in time and prevent the Holocaust specifically, your time would be better well spent killing Henry Ford or Arthur de Gobineau or Martin Luther. Mass murder of Jews in Europe had been common place for centuries. It was a sentiment the Nazis capitalized on, not one they invented.

Similarly, if you wanted to deter shape of European politics that lead to the World Wars, temporal assassination of Bismark or Napoleon III would be far more effective. Those battle lines had been drawn well in advance of 1932.