Objection 1. It would seem that a man who is condemned to death may lawfully defend himself if he can. For it is always lawful to do that to which nature inclines us, as being of natural right, so to speak. Now, to resist corruption is an inclination of nature not only in men and animals but also in things devoid of sense. Therefore if he can do so, the accused, after condemnation, may lawfully resist being put to death.
Objection 2. Further, just as a man, by resistance, escapes the death to which he has been condemned, so does he by flight. Now it is lawful seemingly to escape death by flight, according to Sirach 9:18, "Keep thee far from the man that hath power to kill [and not to quicken]" [The words in the brackets are not in the Vulgate]. Therefore it is also lawful for the accused to resist.
Objection 3. Further, it is written (Proverbs 24:11): "Deliver them that are led to death: and those that are drawn to death forbear not to deliver." Now a man is under greater obligation to himself than to another. Therefore it is lawful for a condemned man to defend himself from being put to death.
On the contrary, The Apostle says (Romans 13:2): "He that resisteth the power, resisteth the ordinance of God: and they that resist, purchase to themselves damnation." Now a condemned man, by defending himself, resists the power in the point of its being ordained by God "for the punishment of evil-doers, and for the praise of the good" [1 Peter 2:14. Therefore he sins in defending himself.
I answer that, A man may be condemned to death in two ways. First justly, and then it is not lawful for the condemned to defend himself, because it is lawful for the judge to combat his resistance by force, so that on his part the fight is unjust, and consequently without any doubt he sins.
Secondly a man is condemned unjustly: and such a sentence is like the violence of robbers, according to Ezekiel 22:21, "Her princes in the midst of her are like wolves ravening the prey to shed blood." Wherefore even as it is lawful to resist robbers, so is it lawful, in a like case, to resist wicked princes; except perhaps in order to avoid scandal, whence some grave disturbance might be feared to arise.
Reply to Objection 1. Reason was given to man that he might ensue those things to which his nature inclines, not in all cases, but in accordance with the order of reason. Hence not all self-defense is lawful, but only such as is accomplished with due moderation.
Reply to Objection 2. When a man is condemned to death, he has not to kill himself, but to suffer death: wherefore he is not bound to do anything from which death would result, such as to stay in the place whence he would be led to execution. But he may not resist those who lead him to death, in order that he may not suffer what is just for him to suffer. Even so, if a man were condemned to die of hunger, he does not sin if he partakes of food brought to him secretly, because to refrain from taking it would be to kill himself.
Reply to Objection 3. This saying of the wise man does not direct that one should deliver a man from death in opposition to the order of justice: wherefore neither should a man deliver himself from death by resisting against justice.
Source: Summa Theologica (New Advent)