A megalomaniac is a person with an abnormal lust for power over others. Many dictators or tyrants around the world are megalomaniacs. A prime example of a megalomaniac is Joseph Stalin, who enjoyed personally wielding his power during his rule over the Soviet Union, which claimed the lives of countless innocent people. Adolf Hitler, however, is not a good example; while it is true that he had a lot of power and was very oppressive, he was a lazy man and liked to delegate his power to other people to carry out vague orders. Typically, the only people who were victims of Hitler's megalomania were his generals and staff, who would be punished or demoted if they failed to achieve their objectives.
Not all megalomaniacs have a lot of power, however. Many are ordinary people who take a somewhat pathetic pleasure in wielding whatever measure of power they may have. This can be as small-scale as, for example, a bartender arbitrarily refusing to serve customers he does not like, or a librarian rigorously enforcing the rules if a reader is a minute late returning a book.