The same divine law which forbids the killing of a human being allows certain exceptions, as when God authorizes killing by a general law or when He gives an explicit commission to an individual for a limited time. Since the agent of authority is but a sword in the hand, and is not responsible for the killing, it is in no way contrary to the commandment, ‘Thou shalt not kill,’ to wage war at God’s bidding, or for the representatives of the State’s authority to put criminals to death, according to law or the rule of rational justice.
Thus, Abraham was not only free from the guilt of criminal cruelty, but even commended for his piety, when he consented to sacrifice his son, not, indeed, with criminal intent but in obedience to God.
One may well ask, also, whether it was not at God’s command that Jeptha killed his daughter when she met him after he had vowed that he would sacrifice to God the first thing he encountered if he returned victorious from battle.
Samson crushed himself and his enemies to death beneath the ruins of a building. He can only be excused on the grounds that the Spirit of the Lord, who wrought miracles through him, had bidden him to do so.
But, apart from such men excepted by the command of a just law in general or of God, the very Source of justice, in a special case, anyone who kills a human being, himself or another, is guilty of murder.
Augustine of Hippo, The City of God, Books I–VII