City of God

However, there are some exceptions made by the divine authority to its own law, that men may not be put to death. These exceptions are of two kinds, being justified either by a general law, or by a special commission granted for a time to some individual. And in this latter case, he to whom authority is delegated, and who is but the sword in the hand of him who uses it, is not himself responsible for the death he deals. And, accordingly, they who have waged war in obedience to the divine command, or in conformity with His laws, have represented in their persons the public justice or the wisdom of government, and in this capacity have put to death wicked men; such persons have by no means violated the commandment, “You shall not kill.”

Abraham indeed was not merely deemed guiltless of cruelty, but was even applauded for his piety, because he was ready to slay his son in obedience to God, not to his own passion. And it is reasonably enough made a question, whether we are to esteem it to have been in compliance with a command of God that Jephthah killed his daughter, because she met him when he had vowed that he would sacrifice to God whatever first met him as he returned victorious from battle. Samson, too, who drew down the house on himself and his foes together, is justified only on this ground, that the Spirit who wrought wonders by him had given him secret instructions to do this.

With the exception, then, of these two classes of cases, which are justified either by a just law that applies generally, or by a special intimation from God Himself, the fountain of all justice, whoever kills a man, either himself or another, is implicated in the guilt of murder.