From St. Thomas's political treatise addressed to the King of Cyprus (http://dhspriory.org/thomas/DeRegno.htm#12), he describes the difference between kings and tyrants:
Quote from: St. Thomas Aquinas, De regno ad regem CypriCHAPTER 12
WHAT PUNISHMENTS ARE IN STORE FOR A TYRANT
 From the above arguments it is evident that stability of power, wealth, honour and fame come to fulfil the desires of kings rather than tyrants, and it is in seeking to acquire these things unduly that princes turn to tyranny. For no one falls away from justice except through a desire for some temporal advantage.
 The tyrant, moreover, loses the surpassing beatitude which is due as a reward to kings and, which is still more serious, brings upon himself great suffering as a punishment. For if the man who despoils a single man, or casts him into slavery, or kills him, deserves the greatest punishment (death in the judgment of men, and in the judgment of God eternal damnation), how much worse tortures must we consider a tyrant deserves, who on all sides robs everybody, works against the common liberty of all, and kills whom he will at his merest whim?
 Again, such men rarely repent; but puffed up by the wind of pride, deservedly abandoned by God for their sins, and besmirched by the flattery of men, they can rarely make worthy satisfaction. When will they ever restore all those things which they have received beyond their just due? Yet no one doubts that they are bound to restore those ill-gotten goods. When will they make amends to those whom they have oppressed and unjustly injured in their many ways?
 The malice of their impenitence is increased by the fact that they consider everything licit which they can do unresisted and with impunity. Hence they not only make no effort to repair the evil they have done but, taking their customary way of acting as their authority, they hand on their boldness in sinning to posterity. Consequently they are held guilty before God, not only for their own sins, but also for the crimes of those to whom they gave the occasion of sin.
 Their sin is made greater also from the dignity of the office they have assumed. Just as an earthly king inflicts a heavier punishment upon his ministers if he finds them traitors to him, so God will punish more severely those whom He made the executors and ministers of His government if they act wickedly, turning God's judgment into bitterness. Hence, in the Book of Wisdom (6:5-7), the following words are addressed to wicked kings: "Because being ministers of His kingdom, you have not judged rightly nor kept the law of justice nor walked according to the will of God, horribly and speedily will He appear to you, for a most severe judgment shall be for them that bear rule; for to him that is little, mercy is granted, but the mighty shall be mightily tormented." And to Nabuchodonosor it is said by Isaiah (14:15-16): "But you shall yet be brought down to hell, into the depth of the pit. They who see you shall turn toward you and gaze on you" as one more deeply buried in punishments.
 So, then, if to kings an abundance of temporal goods is given and an eminent degree of beatitude prepared for them by God, while tyrants are often prevented from obtaining even the temporal goods which they covet, subjected also to many dangers and, worse still, deprived of eternal happiness and destined for most grievous punishment, surely those who undertake the office of ruling must earnestly strive to act as kings towards their subjects, and not as tyrants.
 What has been said hitherto should suffice in order to show what a king is, and that it is good for the multitude to have a king, and also that it is expedient for a ruler to conduct himself towards the multitude of his subjects as a king, not as a tyrant.