"Every empire"? North Korea is not an "Empire". That word has a meaning, and it's not "horrible place to live". Actual empires, like the British Empire, or the pre-1918 German Empire, so on and so forth may have had one person steering the thing, but it was not an absolutism. Even Hitler's Germany was ran largely from the middle-tier of the bureaucracy. That's partly why it was so horrible, you had countless competing fiefdoms with overlapping spheres of competence and authority, each vying to get the Fuehrer's good word.Because every empire has an ultimate ruler people looked to. Hitler, Stalin, Mao, the Kim family of N Korea, etc.
A world-state would probably have a system of government similar to a combination between the American system and the Westminster system. An absolutist world government simply isn't feasible. These only become viable when you have multiple planets, because otherwise your power-base is too liable to fracture.