Tyrant :  (noun)
1: a cruel and oppressive dictator [syn: autocrat, despot]
2: in ancient Greece, a ruler who had seized power without legal right to it
3: any person who exercises power in a cruel way; "his father was a tyrant"

Simon Legree, absolute monarch, absolute ruler, all-powerful ruler, arrogator, autarch, authoritarian, autocrat, bully, caesar, commissar, czar, despot, dictator, disciplinarian, driver, duce, hard master, martinet, oligarch, oppressor, overlord, pharaoh, slave driver, stickler, strong man, totalitarian, usurper, warlord,