319. The fifth commandment is: "Thou shalt not kill". Like most of the other commandments, it expresses a moral principle in a pithy way, so as to impress it on the dullest memory; but it needs to be more fully explained both by reason and by reference to various teachings of the Holy Scriptures and Tradition. In these we are taught that we are not forbidden to kill brute animals; for God said to Noe after the Flood: "Everything that moveth and liveth shall be meat for you; even as the green herbs have I delivered them to you" (Gen. IX, 3). And reason teaches that all lower things are made for man, while man is made for God alone (n. 151). But we are forbidden to kill our fellow-man. For a man is not made for the use of his fellow-men, since all men are equal, having the same nature. God alone has the right to take our life. Therefore no human life can lawfully be destroyed by any man or any body of men, unless God delegate to them His right in the matter. Now there is one case in which God gives to the State the right of intentionally destroying human life; namely, by way of capital punishment for enormous crime; for all nations have always judged so, and in the Old Testament we find this penalty appointed for various offences. St. Paul signifies the same when he tells the Romans that the ruler "bears not the sword in vain, for he is God's minister, an avenger to execute wrath upon him that doth evil" (XIII, 4). The State may also wage a just war. War is a dreadful evil; yet it may at times be necessary to maintain the moral order among nations.
The State then acts in self-defence; but, even so, it has no right to do more harm to the enemy than is strictly necessary, and therefore it should slay none but active combatants. When war is the only means by which a country can maintain its just rights, it is not then reprobated by reason, nor by the Scriptures, nor by the Church.
320. From the principles explained above it is clear that
1. Suicide, that is deliberate self-murder, is always a grievous wrong; for it is a usurpation of the sovereign dominion of God over the life of man.
2. We must take good care of our lives, our limbs, and our health; because they are entrusted to us for the service of our supreme Master. This does not forbid us to expose them to danger when it is necessary to do so in order to attain a higher good.
3. The State alone, not any number of private persons, may inflict the death penalty on a guilty man. Still a private man, when unjustly attacked, may defend life, or limb, or important possessions, by such acts as are strictly necessary for self-defense, even if these acts result in the death of his unjust assailant, when this is the only available means to escape from present danger.
4. No one may promote a war, or volunteer his services in it, unless he is certain that the cause is just, and that the war is the only possible means to secure very important rights. When the justice of the war is doubtful, we are not allowed to expose ourselves to the danger of committing a great wrong by favoring it (n. 301). But drafted soldiers, and those who were enlisted before they suspected the injustice of the war, may presume its justice until it is disproved.
Physicians in particular ought to remember that they can never lawfully procure the death of any human being, or purposely shorten a human life under any pretense whatever.
6. It is never lawful to fight a duel, that is, a combat in which two persons fight with deadly weapons on a pre-arranged plan, unless they act as the champions of two nations at war with each other.
7. The fifth commandment is also violated by gluttony and other excesses which injure health; and chiefly by drunkenness, because, besides injuring health, it takes away what is naturally noblest in man, namely, his intellect and free-will, by which he is made in the image of God; it also stirs up the vilest passions of our nature, and opens wide the flood-gates to all kinds of evil.