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U.S. Catholic Catechism For Adults, U.S.C.C.B., 2006 A.D.

"Self-defense against an unjust aggressor is morally permitted. There is also a moral duty for the defense of others by those who are responsible for their lives. Self-defense or the defense of others as the goal of protecting the person or persons threatened. Once the threat is eliminated, no further action is required. In such situations, the deliberate killing of the aggressor can be permitted only when no other solution is possible. Any response to aggression must be proportionate to the nature of the threat or the act of aggression."

--- United States Catholic Catechism For Adults, Pages 390- 391; or Part III, Ch. 29. © Copyright, United States Conference of Catholic Bishops.

Catechism of the Catholic Church, 1997 A.D.

Legitimate defense

2263 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."65

2264 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.66

2265 Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others. The defense of the common good requires that an unjust aggressor be rendered unable to cause harm. For this reason, those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility.

--- Catechism of the Catholic Church. © Copyright, Libreria Editrice Vaticana.

Evangelium Vitae, Encyclical of Pope St. John Paul II, 1995 A.D.

55. This should not cause surprise: to kill a human being, in whom the image of God is present, is a particularly serious sin. Only God is the master of life! Yet from the beginning, faced with the many and often tragic cases which occur in the life of individuals and society, Christian reflection has sought a fuller and deeper understanding of what God's commandment prohibits and prescribes. 43 There are in fact situations in which values proposed by God's Law seem to involve a genuine paradox. This happens for example in the case of legitimate defence, in which the right to protect one's own life and the duty not to harm someone else's life are difficult to reconcile in practice. Certainly, the intrinsic value of life and the duty to love oneself no less than others are the basis of a true right to self-defence. The demanding commandment of love of neighbour, set forth in the Old Testament and confirmed by Jesus, itself presupposes love of oneself as the basis of comparison: "You shall love your neighbour as yourself " (Mk 12:31). Consequently, no one can renounce the right to self-defence out of lack of love for life or for self. This can only be done in virtue of a heroic love which deepens and transfigures the love of self into a radical self-offering, according to the spirit of the Gospel Beatitudes (cf. Mt 5:38-40). The sublime example of this self-offering is the Lord Jesus himself.

Moreover, "legitimate defence can be not only a right but a grave duty for someone responsible for another's life, the common good of the family or of the State".44 Unfortunately it happens that the need to render the aggressor incapable of causing harm sometimes involves taking his life. In this case, the fatal outcome is attributable to the aggressor whose action brought it about, even though he may not be morally responsible because of a lack of the use of reason. 45

--- Evangelium Vitae. © Copyright, Libreria Editrice Vaticana.

Catechism of Pius X, 1905 A.D.

3 Q. Are there cases in which it is lawful to kill?

A. It is lawful to kill when fighting in a just war; when carrying out by order of the Supreme Authority a sentence of death in punishment of a crime; and, finally, in cases of necessary and lawful defence of one's own life against an unjust aggressor.

--- Catechism of St. Pius X.

Baltimore Catechism, 1891 A.D.

Q. 1276. Under what circumstances may human life be lawfully taken?

A. Human life may be lawfully taken:

1. In self-defense, when we are unjustly attacked and have no other means of saving our own lives; ...

--- Baltimore Catechism.

Douay Catechism, 1649 A.D.

Q. 484. Is it not lawful to kill in any cause?

A. Yes, in a just war, or when public justice requires it: "For the magistrate beareth not the sword without cause." Rom. i. 4. As also in the blameless defence of our own, or our innocent neighbour's life, against an unjust invader.

--- The Douay Catechism of 1649 by Henry Tuberville, D.D. [Mirror 1]

Catechism of Trent, 1566 A.D.

If a man kill another in self-defence, having used every means consistent with his own safety to avoid the infliction of death, he evidently does not violate this Commandment.

--- Catechism of the Council of Trent III, §§ 327-332.