"The same divine authority that forbids the killing of a human being establishes 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.
"The agent who executes the killing does not commit homicide; he is an instrument as is the sword with which he cuts. Therefore, 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 public authority to put criminals to death, according to the law, that is, the will of the most just reason."
-- The City of God, Book 1, chapter 21
