1829. The person who is killed must be a true aggressor, for otherwise the slayer is himself the aggressor and guilty of unjustifiable homicide. Killing is therefore unjust in the following cases:
(a) when the opponent is not using true violence, as when he merely prays and hopes that you may die or be killed;
(b) when he is not using actual violence, as when he is disarmed or helpless, or when he has only threatened to kill you in the future, or to bring you to the gallows by his testimony or vote.
1830. Must one wait, then, until the aggressor has actually attacked, before using self-defense? (a) One need not wait until physical aggression has started (e.g., until the adversary has fired a shot or struck a blow); otherwise self-defense would very often be futile. (b) One should wait until moral aggression has been shown before proceeding to defense; that is, the other party must perform some external act which according to the judgment of a prudent person at the time and place is one with the act of physical aggression, as when an angry man reaches for a gun or knife, or a desperado advances in a threatening manner.
1831. The aggression must also be unjust or contrary to the right of the person attacked. (a) Thus, if the aggression is just, it is not lawful to kill the aggressor. Hence, it is not lawful to kill an officer of the law who is making an arrest, or guarding a prisoner, or leading him to execution. (b) If the aggression is not just, self-defense is lawful. It makes no difference whether the aggression is formally unjust (e.g., when the aggressor attacks you because he wishes to wreak revenge, or because he fears you), or only materially unjust (e.g., when you are a stranger to the aggressor, but he is drunk, or a dope fiend, or a maniac). Similarly, a son may defend himself against his parent, a subject against his superior, a layman against a cleric, an adulterer against the injured husband, a calumniator against the person calumniated; for authority or personal injury suffered does not give these persons the right to inflict by private authority the punishment of death.
1832. Self-defense must be merely a protection of self against future evil and not a punishment of the aggressor for past attacks, for capital punishment belongs to society, not to private persons. Hence, if an aggressor has taken to flight, or has been disarmed, or knocked senseless, or has begged for mercy, it is not permissible to kill him.
1833. Self-defense must be moderate, for injury or the death of a human being is a thing that should be avoided when possible. (a) Thus, the person attacked must not reply with force at all, if this is possible. He should escape, or call a policeman, or throw the weapon out of the window, etc., if these means will suffice. Some authors excuse from flight those who would suffer disgrace if they ran away from danger, such as those who are pugilists or professional fighting men. (b) The person attacked should use only such force as is necessary, if force must be employed. Thus, if the aggressor can be made helpless by the use of gas, permanent bodily harm should not be done him; if he can be subdued with the fists, knife or pistol wounds should not be resorted to; if wounds will suffice to hold him at bay (e.g., by blackjacking), killing should not be resorted to. In the heat of a fight, however, the person assailed sometimes unintentionally goes beyond what moderation requires.
1834. The intention of the person who uses force to repel an unjust aggressor must be good. (a) Thus, as his end he must intend only the preservation of his own life and look upon the death of his neighbor as a misfortune. (b) As the means to this end he must intend only to stop the attack that is being made, not to bring on the death of the aggrcssor. Those who are commissioned by public authority to put a human being to death (e.g., the public executioner or soldiers in time of war) may intend these homicides, since they are means to the common good; but the death of one private person is not a means to the good of another private person, and hence it should not be directly aimed at.
1835. The mind of the person who defends himself against the unjust aggressor must also be free from sinful dispositions, such as hatred and revenge; otherwise he sins against charity. Our Lord reprobated the teaching of the Scribes that it is lawful to return injury for injury in a revengeful spirit, and declared that one should prefer to receive a second blow rather than return a blow for the sake of revenge (Matt., v. 38 sqq.; cfr. also Rom., xii. 19).
1836. When Self-Defense Is Obligatory.—Self-defense is sometimes an obligation. (a) Thus, it is an obligation, if the only factors considered are the life of the guilty aggressor and the life of the innocent person who is assailed; for the life of the innocent should not be sacrificed for the guilty, and charity indicates that the first duty of the person attacked is to himself. (b) It is an obligation, if, on account of circumstances, the person attacked owes it to others to preserve his life—for example, if he is the father of a dependent family, or a public official whose life is very necessary for the welfare of the community, or whose office it is to resist those who menace public security. This is true from the viewpoint of society also, for the world needs the good men it has, while there are too many wicked men already.