St. Thomas Aquinas, II-II, Question 42:
Tyrannical governance is unjust, since it is ordered to the private good of the ruler, not to the common good, as the Philosopher makes clear in the Politics and the Ethics. And so disturbances of such governance does not have the character or rebellion, except, perhaps, in cases where the tyrant’s governance is so inordinately disturbed that the subject people suffer greater harm from the resulting disturbance than from the tyrant’s governance. Rather, tyrants, who by seeking greater domination incite discontent and rebellion in the people subject to them, are the rebels. For governance is tyrannical when ordered to the ruler’s own good to the detriment of the people.