The Catechism of Pope St. Pius X:
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.
The Catechism of the Council of Trent:
THE FIFTH COMMANDMENT : “Thou shalt not kill” . . .
Another kind of lawful slaying belongs to the civil authorities, to whom is entrusted power of life and death, by the legal and judicious exercise of which they punish the guilty and protect the innocent. The just use of this power, far from involving the crime of murder, is an act of paramount obedience to this Commandment which prohibits murder. The end of the Commandment is the preservation and security of human life. Now the punishments inflicted by the civil authority, which is the legitimate avenger of crime, naturally tend to this end, since they give security to life by repressing outrage and violence. Hence these words of David: In the morning I put to death all the wicked of the land, that I might cut off all the workers of iniquity from the city of the Lord.
In like manner, the soldier is guiltless who, actuated not by motives of ambition or cruelty, but by a pure desire of serving the interests of his country, takes away the life of an enemy in a just war.
Furthermore, there are on record instances of carnage executed by the special command of God. The sons of Levi, who put to death so many thousands in one day, were guilty of no sin; when the slaughter had ceased, they were addressed by Moses in these words: You have consecrated your hands this day to the Lord.
The Catechism of St. Thomas Aquinas:
The Execution of Criminals.–Some have held that the killing of man is prohibited altogether. They believe that judges in the civil courts are murderers, who condemn men to death according to the laws. Against this St. Augustine says that God by this Commandment does not take away from Himself the right to kill. Thus, we read: “I will kill and I will make to live.”[5] It is, therefore, lawful for a judge to kill according to a mandate from God, since in this God operates, and every law is a command of God: “By Me kings reign, and lawgivers decree just things.”[6] And again: “For if thou dost that which is evil, fear; for he beareth not the sword in vain. Because he is God’s minister.”[7] To Moses also it was said: “Wizards thou shalt not suffer to live.”[8] And thus that which is lawful to God is lawful for His ministers when they act by His mandate. It is evident that God who is the Author of laws, has every right to inflict death on account of sin. For “the wages of sin is death.”[9] Neither does His minister sin in inflicting that punishment. The sense, therefore, of “Thou shalt not kill” is that one shall not kill by one’s own authority. [10]
- Deut., xxxii. 39.
- Prov., viii. 15.
- Rom., xiii. 4.
- Exod., xxii. 18.
- Rom. vi. 23.
- Killing in a just war and killing by accident are among the other exceptions to this Commandment. The soldier is guiltless who in a just war takes the life of an enemy, provided that he is not actuated by motives of ambition or cruelty, but by a pure desire to serve the interests of his country. . . . Again, death caused, not by intent or design, but by accident, is not murder" (“Roman Catechism,” “loc. cit.,” 5-6).
St. Alphonsus Maria de Liguori, Doctor of the Church:
WHAT DOES THE FIFTH COMMANDMENT FORBID?
. . . It is lawful to put a man to death by public authority: it is even a duty of princes and of judges to condemn to death criminals who deserve it; and it is the duty of the officers of justice to execute the sentence; **God himself wishes malefactors to be punished. **
- It is lawful in self-defence to kill an unjust aggressor, when there is no other means at hand for saving your own life. . . It is also commonly held by theologians, by St. Antonine and by St. Thomas, that it is lawful to kill a robber who, after being admonished to desist, obstinately perseveres in the robbery; and they ground their doctrine on the following passage in Exodus: If a thief be found breaking open a house or undermining it, and be wounded so as to die, he that slew him shall not be guilty of blood." But this decision is to be admitted only when the theft is very considerable ; and, as several theologians hold, only when the owner or his family would be reduced to great want and inconvenience if he permitted the robbery. Theologians also teach that it is lawful to kill a person who attempts to violate your chastity, if you have no other means of preserving it.
- It is lawful to kill enemies in a just war; and even in a war when its justice is doubtful, if you are commanded by your own sovereign.