Today’s Catechism sections begin the discussion of the Fifth Commandment. Supporting material comes from the “Summa Theologica”.

Article 5

THE FIFTH COMMANDMENT

You shall not kill.54

You have heard that it was said to the men of old, "You shall not kill: and whoever kills shall be liable to judgment." But I say to you that every one who is angry with his brother shall be liable to judgment.55

2258 "Human life is sacred because from its beginning it involves the creative action of God and it remains for ever in a special relationship with the Creator, who is its sole end. God alone is the Lord of life from its beginning until its end: no one can under any circumstance claim for himself the right directly to destroy an innocent human being."56

I. Respect for Human Life

The witness of sacred history

2259 In the account of Abel's murder by his brother Cain,57 Scripture reveals the presence of anger and envy in man, consequences of original sin, from the beginning of human history. Man has become the enemy of his fellow man. God declares the wickedness of this fratricide: "What have you done? The voice of your brother's blood is crying to me from the ground. And now you are cursed from the ground, which has opened its mouth to receive your brother's blood from your hand."58

2260 The covenant between God and mankind is interwoven with reminders of God's gift of human life and man's murderous violence:

For your lifeblood I will surely require a reckoning.... Whoever sheds the blood of man, by man shall his blood be shed; for God made man in his own image.59

The Old Testament always considered blood a sacred sign of life.60 This teaching remains necessary for all time.

2261 Scripture specifies the prohibition contained in the fifth commandment: "Do not slay the innocent and the righteous."61 The deliberate murder of an innocent person is gravely contrary to the dignity of the human being, to the golden rule, and to the holiness of the Creator. The law forbidding it is universally valid: it obliges each and everyone, always and everywhere.

2262 In the Sermon on the Mount, the Lord recalls the commandment, "You shall not kill,"62 and adds to it the proscription of anger, hatred, and vengeance. Going further, Christ asks his disciples to turn the other cheek, to love their enemies.63 He did not defend himself and told Peter to leave his sword in its sheath.64

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.

2265 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.66

IN BRIEF

2318 "In [God's] hand is the life of every living thing and the breath of all mankind" (⇒ Job 12:10).

2319 Every human life, from the moment of conception until death, is sacred because the human person has been willed for its own sake in the image and likeness of the living and holy God.

2320 The murder of a human being is gravely contrary to the dignity of the person and the holiness of the Creator.

2321 The prohibition of murder does not abrogate the right to render an unjust aggressor unable to inflict harm. Legitimate defense is a grave duty for whoever is responsible for the lives of others or the common good.

In the “Summa Theologica” (Secunda Secundæ Partis, 64, 7), St. Thomas Aquinas discusses killing in self-defense.

Article 7. Whether it is lawful to kill a man in self-defense?

Objection 1. It would seem that nobody may lawfully kill a man in self-defense. For Augustine says to Publicola (Ep. xlvii): "I do not agree with the opinion that one may kill a man lest one be killed by him; unless one be a soldier, exercise a public office, so that one does it not for oneself but for others, having the power to do so, provided it be in keeping with one's person." Now he who kills a man in self-defense, kills him lest he be killed by him. Therefore this would seem to be unlawful.

Objection 2. Further, he says (De Lib. Arb. i, 5): "How are they free from sin in sight of Divine providence, who are guilty of taking a man's life for the sake of these contemptible things?" Now among contemptible things he reckons "those which men may forfeit unwillingly," as appears from the context (De Lib. Arb. i, 5): and the chief of these is the life of the body. Therefore it is unlawful for any man to take another's life for the sake of the life of his own body.

Objection 3. Further, Pope Nicolas [Nicolas I, Dist. 1, can. De his clericis] says in the Decretals: "Concerning the clerics about whom you have consulted Us, those, namely, who have killed a pagan in self-defense, as to whether, after making amends by repenting, they may return to their former state, or rise to a higher degree; know that in no case is it lawful for them to kill any man under any circumstances whatever." Now clerics and laymen are alike bound to observe the moral precepts. Therefore neither is it lawful for laymen to kill anyone in self-defense.

Objection 4. Further, murder is a more grievous sin than fornication or adultery. Now nobody may lawfully commit simple fornication or adultery or any other mortal sin in order to save his own life; since the spiritual life is to be preferred to the life of the body. Therefore no man may lawfully take another's life in self-defense in order to save his own life.

Objection 5. Further, if the tree be evil, so is the fruit, according to Matthew 7:17. Now self-defense itself seems to be unlawful, according to Romans 12:19: "Not defending [Douay: 'revenging'] yourselves, my dearly beloved." Therefore its result, which is the slaying of a man, is also unlawful.

On the contrary, It is written (Exodus 22:2): "If a thief be found breaking into a house or undermining it, and be wounded so as to die; he that slew him shall not be guilty of blood." Now it is much more lawful to defend one's life than one's house. Therefore neither is a man guilty of murder if he kill another in defense of his own life.

I answer that, Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. Now moral acts take their species according to what is intended, and not according to what is beside the intention, since this is accidental as explained above (43, 3; I-II, 12, 1). Accordingly the act of self-defense may have two effects, one is the saving of one's life, the other is the slaying of the aggressor. Therefore this act, since one's intention is to save one's own life, is not unlawful, seeing that it is natural to everything to keep itself in "being," as far as possible. And yet, though proceeding from a good intention, an act may be rendered unlawful, if it be out of proportion to the end. Wherefore if a man, in self-defense, uses more than necessary violence, it will be unlawful: whereas if he repel force with moderation his defense will be lawful, because according to the jurists [Cap. Significasti, De Homicid. volunt. vel casual.], "it is lawful to repel force by force, provided one does not exceed the limits of a blameless defense." Nor is it necessary for salvation that a man omit the act of moderate self-defense in order to avoid killing the other man, since one is bound to take more care of one's own life than of another's. But as it is unlawful to take a man's life, except for the public authority acting for the common good, as stated above (Article 3), it is not lawful for a man to intend killing a man in self-defense, except for such as have public authority, who while intending to kill a man in self-defense, refer this to the public good, as in the case of a soldier fighting against the foe, and in the minister of the judge struggling with robbers, although even these sin if they be moved by private animosity.

Reply to Objection 1. The words quoted from Augustine refer to the case when one man intends to kill another to save himself from death. The passage quoted in the Second Objection is to be understood in the same sense. Hence he says pointedly, "for the sake of these things," whereby he indicates the intention. This suffices for the Reply to the Second Objection.

Reply to Objection 3. Irregularity results from the act though sinless of taking a man's life, as appears in the case of a judge who justly condemns a man to death. For this reason a cleric, though he kill a man in self-defense, is irregular, albeit he intends not to kill him, but to defend himself.

Reply to Objection 4. The act of fornication or adultery is not necessarily directed to the preservation of one's own life, as is the act whence sometimes results the taking of a man's life.

Reply to Objection 5. The defense forbidden in this passage is that which comes from revengeful spite. Hence a gloss says: "Not defending yourselves--that is, not striking your enemy back."

Footnotes

54 ⇒ Ex 20:13; Cf. ⇒ Deut 5:17.
55 ⇒ Mt 5:21-22.
56 CDF, instruction, Donum vitae, intro. 5.
57 Cf. ⇒ Gen 4:8-12.
58 ⇒ Gen 4:10-11.
59 ⇒ Gen 9:5-6.
60 Cf. ⇒ Lev 17:14
61 ⇒ Ex 23:7.
62 ⇒ Mt 5:21.
63 Cf. ⇒ Mt 5:22-39; ⇒ 5:44.
64 Cf. ⇒ Mt 26:52.
65 St. Thomas Aquinas, STh II-II, 64, 7, corp. art.
66 St. Thomas Aquinas, STh II-II, 64, 7, corp. art.