Aquinas

Now the end which befits a multitude of free men is different from that which befits a multitude of slaves, for the free man is one who exists for his own sake, while the slave, as such, exists for the sake of another. If, therefore, a multitude of free men is ordered by the ruler towards the common good of the multitude, that rulership will be right and just, as is suitable to free men. If, on the other hand, a rulership aims, not at the common good of the multitude, but at the private good of the ruler, it will be an unjust and perverted rulership.