In my opinion, the worst thing to have ever occurred in the world was the campaign of mass shooting carried out by the Nazis in the Soviet Union in 1941 and 1942. Most of the victims were Jews, and so the killings were part of the Final Solution (as it was called when I was in college) or Holocaust (as it is called now) of 1941-1945. The Holocaust itself was but one of a number of massacres carried out in the 20th Century (including, for example, the Turkish slaughters of Armenians during and after World War I, the Stalinist Great Terror of 1937-1938, and the mass executions by the Pol Pot regime in Cambodia in the 1970's.) Still, I think what the German Nazis did in the 18 months after the invasion of Russia is the ne plus ultra of bad things that humans have done to one another. Here is why.
Wilkes, Donald E. Jr., "Vox Nazi, Vox Mortis: The Worst Thing in the World" (1985). Popular Media. Paper 29.