Turning to Christian tradition, we may note that the Fathers and Doctors of the Church are virtually unanimous in their support for capital punishment, even though some of them such as St. Ambrose exhort members of the clergy not to pronounce capital sentences or serve as executioners. To answer the objection that the first commandment forbids killing, St. Augustine writes in The City of God:
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.
In the Middle Ages a number of canonists teach that ecclesiastical courts should refrain from the death penalty and that civil courts should impose it only for major crimes. But leading canonists and theologians assert the right of civil courts to pronounce the death penalty for very grave offenses such as murder and treason. Thomas Aquinas and Duns Scotus invoke the authority of Scripture and patristic tradition, and give arguments from reason.
Giving magisterial authority to the death penalty, Pope Innocent III required disciples of Peter Waldo seeking reconciliation with the Church to accept the proposition: “The secular power can, without mortal sin, exercise judgment of blood, provided that it punishes with justice, not out of hatred, with prudence, not precipitation.” In the high Middle Ages and early modern times the Holy See authorized the Inquisition to turn over heretics to the secular arm for execution. In the Papal States the death penalty was imposed for a variety of offenses. The Roman Catechism, issued in 1566, three years after the end of the Council of Trent, taught that the power of life and death had been entrusted by God to civil authorities and that the use of this power, far from involving the crime of murder, is an act of paramount obedience to the fifth commandment.
In modern times Doctors of the Church such as Robert Bellarmine and Alphonsus Liguori held that certain criminals should be punished by death. Venerable authorities such as Francisco de Vitoria, Thomas More, and Francisco Suárez agreed. John Henry Newman, in a letter to a friend, maintained that the magistrate had the right to bear the sword, and that the Church should sanction its use, in the sense that Moses, Joshua, and Samuel used it against abominable crimes.
Throughout the first half of the twentieth century the consensus of Catholic theologians in favor of capital punishment in extreme cases remained solid, as may be seen from approved textbooks and encyclopedia articles of the day. The Vatican City State from 1929 until 1969 had a penal code that included the death penalty for anyone who might attempt to assassinate the pope. Pope Pius XII, in an important allocution to medical experts, declared that it was reserved to the public power to deprive the condemned of the benefit of life in expiation of their crimes.
Avery Cardinal Dulles, “Catholicism & Capital Punishment” (2001)