A king is chosen by either being chosen in an election, or they were the first born in a dynasty. They run and rule the empire that is theirs. A tyrant on the other hand was in ancient Greece, someone who takes power by force and rules with total authority. Tyrants in Ancient Greece were used to overthrow the nobles that had stolen the hoplites/farmers land or hogged wealthy land. Usually farmers who were hoplites owed money to the nobles and then to pay off the debt would sell themselves in to slavery. Today a tyrant is a harsh, oppressive ruler, but ancient Greece ones were wise and fair. Kings were rulers over a nation and were usually unfair