On the contrary, Augustine says (De Civ. Dei i) [*Can. Quicumque percutit, caus. xxiii, qu. 8]: "A man who, without exercising public authority, kills an evil-doer, shall be judged guilty of murder, and all the more, since he has dared to usurp a power which God has not given him."
I answer that, As stated above (A. 2), it is lawful to kill an evildoer in so far as it is directed to the welfare of the whole community, so that it belongs to him alone who has charge of the community's welfare. Thus it belongs to a physician to cut off a decayed limb, when he has been entrusted with the care of the health of the whole body. Now the care of the common good is entrusted to persons of rank having public authority: wherefore they alone, and not private individuals, can lawfully put evildoers to death.
Reply Obj. 1: The person by whose authority a thing is done really does the thing as Dionysius declares (Coel. Hier. iii). Hence according to Augustine (De Civ. Dei i, 21), "He slays not who owes his service to one who commands him, even as a sword is merely the instrument to him that wields it." Wherefore those who, at the Lord's command, slew their neighbors and friends, would seem not to have done this themselves, but rather He by whose authority they acted thus: just as a soldier slays the foe by the authority of his sovereign, and the executioner slays the robber by the authority of the judge.
Reply Obj. 2: A beast is by nature distinct from man, wherefore in the case of a wild beast there is no need for an authority to kill it; whereas, in the case of domestic animals, such authority is required, not for their sake, but on account of the owner's loss. On the other hand a man who has sinned is not by nature distinct from good men; hence a public authority is requisite in order to condemn him to death for the common good.
Reply Obj. 3: It is lawful for any private individual to do anything for the common good, provided it harm nobody: but if it be harmful to some other, it cannot be done, except by virtue of the judgment of the person to whom it pertains to decide what is to be taken from the parts for the welfare of the whole. _______________________
FOURTH ARTICLE [II-II, Q. 64, Art. 4]
Whether It Is Lawful for Clerics to Kill Evil-doers?
Objection 1: It would seem lawful for clerics to kill evil-doers. For clerics especially should fulfil the precept of the Apostle (1 Cor. 4:16): "Be ye followers of me as I also am of Christ," whereby we are called upon to imitate God and His saints. Now the very God whom we worship puts evildoers to death, according to Ps. 135:10, "Who smote Egypt with their firstborn." Again Moses made the Levites slay twenty-three thousand men on account of the worship of the calf (Ex. 32), the priest Phinees slew the Israelite who went in to the woman of Madian (Num. 25), Samuel killed Agag king of Amalec (1 Kings 15), Elias slew the priests of Baal (3 Kings 18), Mathathias killed the man who went up to the altar to sacrifice (1 Mac. 2); and, in the New Testament, Peter killed Ananias and Saphira (Acts 5). Therefore it seems that even clerics may kill evil-doers.
Obj. 2: Further, spiritual power is greater than the secular and is more united to God. Now the secular power as "God's minister" lawfully puts evil-doers to death, according to Rom. 13:4. Much more therefore may clerics, who are God's ministers and have spiritual power, put evil-doers to death.
Obj. 3: Further, whosoever lawfully accepts an office, may lawfully exercise the functions of that office. Now it belongs to the princely office to slay evildoers, as stated above (A. 3). Therefore those clerics who are earthly princes may lawfully slay malefactors.