Alan Bullock, the British historian who wrote the first comprehensive biography of Adolf Hitler, in 1952, considered the Nazi leader “the most evil man in history”. It is a view that has endured, though history has no shortage of mass murderers. Stalin, Mao, King Leopold of Belgium, Winston Churchill, Pol Pot, Mengistu Haile Mariam of Ethiopia, and, going back in history, rulers such as Genghis Khan and Timur have caused millions of deaths. That didn’t prevent British politician Jack Aaron from recently calling the German dictator “brilliant”. Or stop Carl Paladino, a Republican candidate for the US Congress, from saying...