Idem, On the Epistle to the Galatians: He who smites the wicked because they are wicked and whose reason for the murder is that he may slay the base, is a servant of the Lord.
Idem, on Jeremiah: For the punishment of homicides, impious persons and poisoners is not bloodshed, but serving the law.
Cyprian, in the Ninth Kind of Abuse: The King ought to restrain theft, punish deeds of adultery, cause the wicked to perish from off the face of the earth, refuse to allow parricides and perjurers to live.
Augustine: Although it is manslaughter to slaughter a man, a person may sometimes be slain without sin. For both a soldier in the case of an enemy and a judge or his official in the case of a criminal, and the man from whose hand, perhaps without his will or knowledge, a weapon has flown, do not seem to me to sin, but merely to kill a man.
Likewise: The soldier is ordered by law to kill the enemy, and if he shall prove to have refrained from such slaughter, he pays the penalty at the hands of his commander. Shall we not go so far as to call these laws unjust or rather no laws at all? For that which was not just does not seem to me to be a law.
Idem, on Exodus ch. xxvii: The Israelites committed no theft in spoiling the Egyptians, but rendered a service to God at his bidding, just as when the servant of a judge kills a man whom the law hath ordered to be killed; certainly if he does it of his own volition he is a homicide, even though he knows that the man whom he executes ought to be executed by the judge.
Idem, on Leviticus ch. lxxv: When a man is justly put to death, the law puts him to death, not thou.
Idem, Bk. I of the "City of God": Thou shall not kill, except in the case of those whose death God orders, or else when a law hath been passed to suit the needs of the time and express command hath been laid upon a person. But he does not kill who owes service to the person who gives him his orders, for he is as it were a mere sword for the person who employs his assistance.
Likewise: When a soldier, in obedience to the power under which he is legitimately placed, kills a man, by no law of the state is he accused of murder; nay if he has not done it, he is accused of desertion and insubordination. But if he had acted under his own initiative and of his own will, he would have incurred the charge of shedding human blood. And so he is punished if he does not do when ordered that for which he would receive punishment if he did it without orders.
Idem, to Publicola: Counsel concerning the slaying of men pleaseth me not, that none may be slain by them, unless perhaps a man is a soldier or in a public office, so that he does the deed not in his own behalf, but for others and for the state, accepting power legitimately conferred, if it is consonant with the task imposed on him.