“The same divine law which forbids the killing of a human being allows certain exceptions, as when God authorizes killing by a general law or when He gives an explicit commission to an individual for a limited time. Since the agent of authority is but a sword in the hand, and is not responsible for the killing, it is in no way contrary to the commandment, “Thou shalt not kill” to wage war at God’s bidding, or for the representatives of the State’s authority to put criminals to death, according to law or the rule of rational justice.”
(St. Augustine, The City of God)
“Concerning secular power we declare that without mortal sin it is possible to exercise a judgment of blood as long as one proceeds to bring punishment not in hatred but in judgment, not incautiously but advisedly.”
(Pope Innocent III in Denzinger 425)
“Therefore if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and advantageous that he be killed in order to safeguard the common good, since ‘a little leaven corrupteth the whole lump’ (1 Cor. 5:6).”
(St. Thomas Aquinas, Summa Theologica, Part II-II, Q. 64, art. 2.)
“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.”
Catechism of the Council of Trent, commenting on the 5th commandment