Adolf Hitler was the leader of Nazi Germany from 1934 to 1945. He's mostly responsible for World War II, killed several million Jews, Gypsies, Homosexuals, and Mentally or Physically Retarded people in death camps, and had an awful mustache. Widely considered to be one of the most evil people in history.
Naturally, the Internet jokes about this constantly. See also: Godwin's Law.
The Secret Taco Nazi was based on Hitler and was used by CivGeneral to stop Chuck the Freaking Alien until it was eaten, and then erased from existence by a Grandfather paradox triggered by Stylesrj's accidental use of time travel that killed Hitler.
He is also easely angered over just about everything, a comical quirk carried over into DYOS.