Human rights activists in many countries where capital punishment is in vogue, have been advocating repeal of the punishment stating inter alia that that only God is empowered to take a human life. I wish to know what the Catholic Church's official views – both for and against – on capital punishment especially in the form of a deterrent against prospective criminals, are.
The Catholic Church does not forbid capital punishment.
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. (Ps. 100:8).
- By Man Shall His Blood Be Shed: A Catholic Defense of Capital Punishment by Edward Feser & Joseph Bessette (2017)
As of August 2018, the Church teaches that "the death penalty is inadmissible" and should be abolished worldwide
Pope Francis announced on August 2, 2018 that number 2267 of the Catechism of the Catholic Church would be changed to say the following (emphasis mine):
Recourse to the death penalty on the part of legitimate authority, following a fair trial, was long considered an appropriate response to the gravity of certain crimes and an acceptable, albeit extreme, means of safeguarding the common good
Today, however, there is an increasing awareness that the dignity of the person is not lost even after the commission of very serious crimes. In addition, a new understanding has emerged of the significance of penal sanctions imposed by the state. Lastly, more effective systems of detention have been developed, which ensure the due protection of citizens but, at the same time, do not definitively deprive the guilty of the possibility of redemption.
Consequently, the Church teaches, in the light of the Gospel, that “the death penalty is inadmissible because it is an attack on the inviolability and dignity of the person”, and she works with determination for its abolition worldwide.
The full letter to the bishops elaborated upon the reasoning for the change:
- The new revision of number 2267 of the Catechism of the Catholic Church, approved by Pope Francis, situates itself in continuity with the preceding Magisterium while bringing forth a coherent development of Catholic doctrine. The new text, following the footsteps of the teaching of John Paul II in Evangelium vitæ, affirms that ending the life of a criminal as punishment for a crime is inadmissible because it attacks the dignity of the person, a dignity that is not lost even after having committed the most serious crimes. This conclusion is reached taking into account the new understanding of penal sanctions applied by the modern State, which should be oriented above all to the rehabilitation and social reintegration of the criminal. Finally, given that modern society possesses more efficient detention systems, the death penalty becomes unnecessary as protection for the life of innocent people. Certainly, it remains the duty of public authorities to defend the life of citizens, as has always been taught by the Magisterium and is confirmed by the Catechism of the Catholic Church in numbers 2265 and 2266.
 Cf. Vincent of Lérins, Commonitorium, cap. 23: PL 50, 667-669. In reference to the death penalty, treating the stipulations of the precepts of the Decalogue, the Pontifical Biblical Commission spoke of the “refinement” of the moral positions of the Church: “In the course of history and of the development of civilization, the Church too, meditating on the Scriptures, has refined her moral stance on the death penalty and on war, which is now becoming more and more absolute. Underlying this stance, which may seem radical, is the same anthropological basis, the fundamental dignity of the human person, created in the image of God.” (The Bible and Morality: Biblical Roots of Christian Conduct, 2008, n. 98).
In short, the explanation states that the new text:
- Is consistent with previous teachings of the Magesterium
- Asserts basic human dignity that is not lost even for grievous crimes
- Reflects how society has changed from punishing and excluding criminals to rehabilitating and reintegrating them
- Recognizes that the death penalty is largely unnecessary with modern prison systems
The online version of the Catechism of the Catholic Church had at the end of May 2018...
2266 The State's effort to contain the spread of behaviors injurious to human rights and the fundamental rules of civil coexistence corresponds to the requirement of watching over the common good. Legitimate public authority has the right and duty to inflict penalties commensurate with the gravity of the crime. the primary scope of the penalty is to redress the disorder caused by the offense. When his punishment is voluntarily accepted by the offender, it takes on the value of expiation. Moreover, punishment, in addition to preserving public order and the safety of persons, has a medicinal scope: as far as possible it should contribute to the correction of the offender.⁶⁷
2267 The traditional teaching of the Church does not exclude, presupposing full ascertainment of the identity and responsibility of the offender, recourse to the death penalty, when this is the only practicable way to defend the lives of human beings effectively against the aggressor. "If, instead, bloodless means are sufficient to defend against the aggressor and to protect the safety of persons, public authority should limit itself to such means, because they better correspond to the concrete conditions of the common good and are more in conformity to the dignity of the human person. "Today, in fact, given the means at the State's disposal to effectively repress crime by rendering inoffensive the one who has committed it, without depriving him definitively of the possibility of redeeming himself, cases of absolute necessity for suppression of the offender 'today ... are very rare, if not practically non-existent.'[John Paul II, Evangelium vitae 56.]
⁶⁷ Cf Luke 23:40–43
This is different to my printed copy:
[No specific heading: the last was Legitimate defence]
2266 Preserving the common good of society requires rendering the aggressor unable to inflict harm. For the reason the traditional teaching of the Church has acknowledged as well-founded the right and duty of legitimate public authority to punish malefactors by means of penalties commensurate with the gravity of the crime, not excluding, in cases of extreme gravity, the death penalty. For analogous reasons those holding authority have the right to repel by armed force aggressors against the community in their charge.
The primary effect of punishment is to redress the disorder caused by the offence. When his punishment is voluntarily accepted by the offender, it takes on the value of expiation. Moreover, punishment has the effect of preserving public order and the safety of persons. Finally punishment has a medicinal value; as far as possible it should contribute to the correction of the offender.⁶⁷
2267 If bloodless means are sufficient to defend human lives against an aggressor and to protect public order and the safety of persons, public authority should limit itself to such means, because they better correspond to the concrete conditions of the common good and are more in conformity to the dignity of the human person.
⁶⁷ Cf Luke 23:40–43
I don't know when the text was changed (possibly around October 2017 when Pope Francis spoke out against capital punishment). However, both the older and the May 2018 text allow it.