"The fact that the evil ones, as long as they live, can be corrected from their errors does not prohibit that they may be justly executed, for the danger which threatens from their way of life is greater and more certain than the good which may be expected from their improvement.
They also have at that critical point of death the opportunity to be converted to God through repentance. And if they are so obstinate that even at the point of death their heart does not draw back from malice, it is possible to make a quite probable judgment that they would never come away from evil.”
". . . Every part is directed to the whole, as imperfect to perfect, wherefore every part exists naturally for the sake of the whole. For this reason we see that if the health of the whole human body demands the excision of a member, because it became putrid or infectious to the other members, it would be both praiseworthy and healthful to have it cut away. Now every individual person is related to the entire society as a part to the whole. Therefore if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and healthful that he be killed in order to safeguard the common good, since "a little leaven corrupteth the whole lump” (1 Cor. 5:6).
. . . 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."
St. Thomas Aquinas, Doctor of the Church
St. Thomas Aquinas’s Summa Theologica was placed on the altar next to the Bible at the Council of Trent.
St. Thomas is echoed by the other doctors of the Church on this matter. St. Alphonsus, St. Augustine, St. Robert Bellarmine, etc. . .
The teaching of the Church has always been that the death penalty is acceptable, and for a variety of reasons, including for the sake of punishment, rather than alone for the protection of society.
A pope may inject his personal opinions into official declarations as to what is suitable for the times, but the timeless teaching of the Church does not change based upon these opinions. Also, whoever is the more recent and most modern Pope expressing an opinion does not mean simply because of the timing that it is a superior opinion to that of past popes, especially in the light of the majority’s teachings.
The doctrine of the Church does not change.
To reject the death penalty entirely at all times and places, and that it can be delegated to authorities in the government is in fact to reject the teachings of the Church.