megalomaniac

noun

meg·​a·​lo·​ma·​ni·​ac -ˈmā-nē-ˌak 

: one affected with or exhibiting megalomania

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Since the Greek root megalo- means "large", someone who is megalomaniacal has a mental disorder marked by feelings of personal grandeur. Megalomania has probably afflicted many rulers throughout history: The Roman emperor Caligula insisted that he be worshipped as a living god. Joseph Stalin suffered from the paranoia that often accompanies megalomania, and had thousands of his countrymen executed as a result. J.-B. Bokassa, dictator of a small and extremely poor African nation, proclaimed himself emperor of the country he renamed the Central African Empire. And even democratically elected leaders have often acquired huge egos as a result of public acclaim. But megalomaniac is generally thrown around as an insult and rarely refers to real mental illness.