The word ‘tyrant’ is perhaps used too lightly. It is questionable whether any of the Plantagenets qualify when compared to this man:
“Galeazzo Sforza (Duke of Milan) is also known to have had a cruel streak. He was a notorious womanizer who often passed his women on to his courtiers once he was tired of them. Sforza once had a poacher executed by forcing him to swallow an entire hare (with fur intact), and had another man nailed alive to his coffin. He also had a priest who predicted a short reign for Sforza punished by being starved to death. This made him many enemies in Milan. It was also said of Galeazzo Sforza that he had raped the wives and daughters of numerous Milanese nobles, that he took sadistic pleasure in devising tortures for men who had offended him, and that he enjoyed pulling apart the limbs of his enemies with his own hands.” (Wiki)
That sounds like a real tyrant to me.