Acruel and unjust ruler is often known as a

Tyrant

Explanation:

Etymologically, the word tyrant means an illegitimate ruler who uses oppressive powers. A Tyrant is a ruler who is unjust and cruel towards his subjects. He uses brutal and harsh methods to govern the state and the people. They use powers over the people. This results in the victimization of the subjects and suppression of their desires and needs.

classical republicanism, also known as civic humanism, is a form of republicanism  developed in the renaissance inspired by the governmental forms and writings of  classical  antiquity, especially such  classical  writers as aristotle, polybius, and cicero.

good question. i'll be looking into that. (this is a joke sir)