From the Catechism of Saint 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.” * *
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.” 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.” To Moses also it was said: “Wizards thou shalt not suffer to live.” 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.” 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.
From the Catechism of the Council of Trent* *:
Execution of Criminals* *
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. **
From the Catechism of Saint Pius X* *:
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 defense of one’s own life against an unjust aggressor.
From the Catechism of the Catholic Church:* *
**2267 *** *Assuming that the guilty party’s identity and responsibility have been fully determined, **the traditional teaching of the Church does not exclude recourse to the death penalty, if this is the only possible way of effectively defending human lives against the unjust aggressor. **If, however, non-lethal means are sufficient to defend and protect people’s safety from the aggressor, authority will limit itself to such means, as these are more in keeping with the concrete conditions of the common good and are more in conformity to the dignity of the human person. Today, in fact, as a consequence of the possibilities which the state has for effectively preventing crime, by rendering one who has committed an offense incapable of doing harm - without definitely taking away from him the possibility of redeeming himself - the cases in which the execution of the offender is an absolute necessity “are very rare, if not practically non-existent.”
*
And in order to clarify that the excerpt in red is a non-binding opinion of the late pontiff rather than a doctrinal statement: *
From Worthiness to Receive Holy Communion* * by Cardinal Joseph Ratzinger
- Not all moral issues have the same moral weight as abortion and euthanasia. For example, if a Catholic were to be at odds with the Holy Father on the application of capital punishment or on the decision to wage war, he would not for that reason be considered unworthy to present himself to receive Holy Communion. * ***While the Church exhorts civil authorities to seek peace, not war, and to exercise discretion and mercy in imposing punishment on criminals, it may still be permissible to take up arms to repel an aggressor or to have recourse to capital punishment. **There may be a legitimate diversity of opinion even among Catholics about waging war and applying the death penalty, but not however with regard to abortion and euthanasia.
*The Catholic Church does not oppose the death penalty and never has.Individual Catholics are free to personally oppose the death penalty or to support it, but it is erroneous to claim that the Church officially and absolutely opposes it. Abortion and euthanasia are opposed - the death penalty is not.
Catholics opposed to the death penalty can argue from a personal humanitarian perspective, but not from a universally binding doctrinal one.