Tyranny has been pertinent through history for centuries but due to recent 20th century history, many of the world's most infamous tyrants have spawned and are known for their tyrannies.
Adolf Hitler was a German politician, Chancellor, then Dictator from 1934-1945 and believed in racial superiority and a totalitarian regime. By far, one of Hitler's worst actions he did as a dictator was create the controlled and planned slaughter of millions of Jewish people in an event known as The Holocaust. This further hammers in the nail of his beliefs of a perfect race and a perfect world he was attempting to create, by all means necessary; even if it required genocide.
Joseph Stalin was a dictator of the Soviet Union during 1929-1953 and had an extremely tyrannical rule where whoever opposed him was killed. One of his most blatant actions to make his society more oppressive was to rewrite Russian history books to put him in a more prominent role and to place him in a positive connotation.