Defensive homicide is not forbidden when anyone, for the purpose of defending his own life against a violent and unjust aggressor (keeping within the limits of lawful protection), kills another. To be considered as lawful protection, it is necessary: (1) that the aggressor unjustly assails and falls upon us; (2) that the defender be placed beyond all blame, while every other way of escaping morally by speaking or flying or yielding is shut against him; (3) that the defense be made during the very attack and not after it is over; (4) that nothing is done by him either under the impulse of anger or with the feeling and desire of revenge, but with the sole intention of defending himself.
XVI. The reason is clear. Although it is not lawful to return like for like and to avenge oneself, still to repel force by force and to defend oneself belongs to natural and perpetual right (especially where the aggression is simply violent and destitute of all public authority) even unto the slaying of the aggressor (although not intended by itself, but inasmuch as we cannot otherwise defend our lives and free ourselves from his unjust oppression). Nor do civil laws alone approve this, as is evident from the Codex to the Cornelian law and to the Aquilian law: “All laws and all rights allow the repelling of force by force” (cf. Corpus Iuris Civilis, I: Digesta 48.8 [“Ad legem Corneliam de sicariis”] [ed. P. Krueger, 1955], pp. 852–53 and ibid., 9.2.45 [“Ad legem Aquiliam”], p. 162). But God himself is found to have intimated this clearly in the law where a case of private defense is set forth from which a judgment can be formed concerning the practice of that law: “If a thief be found breaking up” (in the very act) “and be smitten that he die, there shall no blood be shed for him” (Ex. 22:2). If the sun be risen upon him, there shall be blood shed for him if doubtless the slayer could discover that he had come only for the purpose of stealing and not of killing.
XVII. However such defense is wrongfully extended to the preservation or recovery of honor (oftentimes imaginary), whose idol the devil (who is a murderer from the beginning) has set up in the world that offering may be made to it with human blood; both because honor can be recovered, but life never; and because such slaughter would pertain not to lawful defense, but to unlawful revenge. But it is properly referred: (1) To the defense of life, whether our own or our neighbor’s, especially when they are bound to us by a somewhat closer tie (as our parents, wives, children, friends and the like). For he who does not repel an injury from another when he can is as much to blame as he who commits it. Still the person acting can be such, as is (for example) a father or a prince, so that it is more becoming for the one attacked to suffer death itself than to repel injury by such a defense. (2) It is referred to the defense of chastity, either our own or another’s (as the examples of brave virgins stand forth, who killed those attempting to violate their chastity, when they could in no other way escape). Just as many laws permit the father or the husband to kill with impunity the violator of a daughter or wife taken in the act (ep’ autophorō).
XVIII. Blameless protection is not prohibited in Rom. 12:19, but private revenge (as the words themselves show: mē heautous ekdikountes). Nor does he who justly defends his own life do it by private undertaking, but by the public authority of the law of nature. The injunctions to love our enemies do not take away the necessary defense of life because the foundation of the love of neighbors is the love of ourselves. The passage in Mt. 26:52 in which our Lord orders Peter, “Put up again thy sword into his place: for all they that take the sword shall perish with the sword” does not take away just self-defense. This had the appearance not so much of defense (which would have been useless against so great a multitude) as of revenge. Again, he had not waited for the command of the Lord (who had no need of such a defender), but acted precipitately.