We do not, because we have heard that, “Thou shalt not kill,” deem it therefore unlawful to pluck a twig, according to the error of the Manichees, nor consider it to extend to irrational brutes; by the most righteous ordinance of the Creator their life and death is subservient to our needs.
There remains, therefore, only man of whom we can understand it, and that not any other man, nor you only; for he who kills himself does nothing else but kill a man. Yet have not they in any way done contrary to this commandment who have waged wars under God’s authority, or they who charged with the administration of civil power have by most just and reasonable orders inflicted death upon criminals. Also Abraham was not charged with cruelty, but even received the praise of piety, for that he was willing to obey God in slaying his son.
Those are to be excepted from this command whom God commands to be put to death, either by a general law given, or by particular admonition at any special time. For he is not the slayer who ministers to the command, like a hilt to one smiting with a sword, nor is Samson otherwise to be acquitted for destroying himself along with his enemies, than because he was so instructed privily of the Holy Spirit, who through him wrought the miracles.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: City of God, book 1, ch. 20