Nowadays, a quick way to get yourself in the news is to compare someone to Hitler. But, prior to Hitler, who was generally accepted to be history's most evil person? It's a complicated question because, as Brian Palmer explains in Slate, "hatred was more local and short-lived before World War II"—meaning people didn't tend to go around comparing political leaders to Genghis Khan or Attila the Hun. Different regions did have their own favorite—and sometimes unlikely—boogeymen, like Abraham Lincoln during the Civil War.
The historical figure who comes the closest is the Egypt's Pharaoh of Exodus, who refused to let the Hebrew people go even as plagues rained down. Thomas Paine used a Pharaoh comparison in the run-up to the Revolutionary War; abolitionists did so during the Civil War; even Martin Luther King Jr. used Pharaoh references. And speaking of Abraham Lincoln? Some Confederates referred to him as "a modern Pharaoh." (Read more Hitler stories.)