1837. Sometimes self-defense is not obligatory. (a) Thus, it is merely optional, when no duty to others commands self-defense and a divine counsel invites one to omit it (see 1169). Hence, if the assailant is certainly in mortal sin, while the person assailed is certainly in the state of grace, it would be very commendable to die rather than kill the assailant, in order to grant him time for repentance. But a case of this kind is rather theoretical than practical, for how could one be sure that the assailant would profit by the opportunity allowed him of doing penance? At any rate, the sacrifice is optional, for the aggressor is either formally unjust, and hence not in extreme spiritual need, or only materially unjust, and it will be uncertain whether he is in spiritual need or whether, if he is in such need, the respite will be used by him (see 1165 sqq.). (b) Self-defense is unlawful according to some, if the life of the assailant is necessary for the common good, and the life of the person assailed is not necessary. But this would be a very rare case.
1838. Defense of Neighbor’s Life.—The principles on defense of one’s own life against an unjust aggressor, even at cost of the latter’s life, may be applied to the life of an innocent third party.
(a) Thus, it is necessary to defend the innocent person, even if the aggressor has to be killed, when one is bound to give this protection by natural duty (e.g., because the innocent person is one’s child or father and the aggressor is not a relative), or by contract (e.g., because one is a hired bodyguard or policeman).
(b) It is lawful to defend the innocent person, even if the aggressor has to be killed, and even though there is no duty of nature or contract to give this protection (Exod., ii, 12). But it is disputed whether it is also necessary to do this. The affirmative opinion calls attention to the extreme bodily need of the innocent party, the negative to the extreme spiritual need of the aggressor. It is not necessary to risk one’s life in order to protect the life of the innocent party, unless the public safety is in peril, or one has undertaken this obligation (see 1169).
1839. A private individual may defend life at the cost of the life of an unjust aggressor, because he is obliged or permitted to protect the life that has more of a claim on him. He may also defend certain other most important goods that belong to him or to his neighbor, even if need be at the cost of the unjust aggressor’s life, because the common good is more valuable than the life of the aggressor, and the defense of those goods is bound up with the common good. Thus, if it were not permissible to defend valuable property even to the extent of killing a burglar, criminals would be encouraged, peaceful citizens would be at a disadvantage, and the public security would greatly suffer. Among the goods now spoken of are goods of fortune and goods of body. It is not always obligatory, however, to exercise the right of extreme self-defense (e.g., in case of violation, provided no consent is given the deed).
1840. Defense of Goods of Fortune Against an Unjust Aggressor.—(a) If the attack is equivalent to an attack on life (e.g., the aggressor wishes to take the last loaf from a starving man or the plank from a drowning man), or if it seems to be an attack on life (e.g., the burglar enters a room as if he meant to kill), the killing of the unjust aggressor is not unlawful, as is clear from the previous paragraphs.
(b) If the attack is made on goods of fortune only, but they are of great value and actually possessed, the question is disputed. According to some, killing in this case is unlawful, because life is more valuable than property; but the common opinion is that killing is lawful, both because Scripture excuses the person who kills the nocturnal robber (Exod., xxii. 2), and because the public safety is at stake and therefore justifies extreme measures.
(c) If the attack is made only on goods of fortune, and they are not yet possessed (e.g., a legacy one hopes to obtain) or have only a small value (e.g., one gold piece), killing is unlawful; for there is no proportion between external goods that are only hoped for or that are of minor importance and the life of a human being. Pope Innocent XI condemned the teaching that one may use homicidal defense to protect a coin or the prospect of receiving an office.
1841. Defense of Bodily Purity Against an Unjust Aggressor.—(a) If the attack is equivalent to an attack on life, or seems to be an attack on life, self-defense even by killing is lawful, and hence it may be permissible to kill one who is attempting rape.
(b) If the attack is on bodily purity only, but _per actum consummatum luxuriae_, the question is controverted. One opinion is that killing may not be resorted to, for the aggressor cannot take away purity of soul, and the purity of body that he violates is less good than life. The opposite opinion holds that killing may be employed in self-defense, since bodily purity has a higher value than even notable goods of fortune, especially as violation is usually accompanied by spiritual damage or disgrace of family, etc.; and the public interest demands that such outrages be sternly suppressed on account of the strong inclination of many persons to commit them.