Self-defence
May a person defend themselves, and how far?
The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. "The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor.... the one is intended, the other is not." St. Thomas Aquinas, STh II-II, 64, 7, corp. art.
Love toward oneself remains a fundamental principle of morality. Therefore it is legitimate to insist on respect for one's own right to life. Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow:
If a man in self-defense uses more than necessary violence, it will be unlawful: whereas if he repels force with moderation, his defense will be lawful.... Nor is it necessary for salvation that a man omit the act of moderate self-defense to avoid killing the other man, since one is bound to take more care of one's own life than of another's.
Legitimate defense can be not only a right but a grave duty for someone responsible for another's life. Preserving the common good requires rendering the unjust aggressor unable to inflict harm. To this end, those holding legitimate authority have the right to repel by armed force aggressors against the civil community entrusted to their charge. St. Thomas Aquinas, STh II-II, 64, 7, corp. art.
The production and the sale of arms affect the common good of nations and of the international community. Hence public authorities have the right and duty to regulate them. the short-term pursuit of private or collective interests cannot legitimate undertakings that promote violence and conflict among nations and compromise the international juridical order.