Joseph Cardinal Ratzinger, while Cardinal Prefect of the Congregation for the Doctrine of the Faith, quoted from a memorandum that was sent by Cardinal Ratzinger to Cardinal McCarrick and was made public in the first week of July 2004. See Worthiness to Receive Holy Communion - General Principles. This quote comes from item #3.

To the Bishops Priests and Deacons Men and Women religious lay Faithful and all People of Good Will on the Value and Inviolability of Human Life...
54. As explicitly formulated, the precept "You shall not kill" is strongly negative: it indicates the extreme limit which can never be exceeded. Implicitly, however, it encourages a positive attitude of absolute respect for life; it leads to the promotion of life and to progress along the way of a love which gives, receives and serves. The people of the Covenant, although slowly and with some contradictions, progressively matured in this way of thinking, and thus prepared for the great proclamation of Jesus that the commandment to love one's neighbour is like the commandment to love God; "on these two commandments depend all the law and the prophets" (cf. Mt 22:36-40). Saint Paul emphasizes that "the commandment ... you shall not kill ... and any other commandment, are summed up in this phrase: ?You shall love your neighbour as yourself' " (Rom 13:9; cf. Gal 5:14). Taken up and brought to fulfilment in the New Law, the commandment "You shall not kill" stands as an indispensable condition for being able "to enter life" (cf. Mt 19:16-19). In this same perspective, the words of the Apostle John have a categorical ring: "Anyone who hates his brother is a murderer, and you know that no murderer has eternal life abiding in him" (1 Jn 3:15).
From the beginning, the living Tradition of the Church-as shown by the Didache, the most ancient non-biblical Christian writing-categorically repeated the commandment "You shall not kill": "There are two ways, a way of life and a way of death; there is a great difference between them... In accordance with the precept of the teaching: you shall not kill ... you shall not put a child to death by abortion nor kill it once it is born ... The way of death is this: ... they show no compassion for the poor, they do not suffer with the suffering, they do not acknowledge their Creator, they kill their children and by abortion cause God's creatures to perish; they drive away the needy, oppress the suffering, they are advocates of the rich and unjust judges of the poor; they are filled with every sin. May you be able to stay ever apart, o children, from all these sins!". 42
As time passed, the Church's Tradition has always consistently taught the absolute and unchanging value of the commandment "You shall not kill". It is a known fact that in the first centuries, murder was put among the three most serious sins-along with apostasy and adultery-and required a particularly heavy and lengthy public penance before the repentant murderer could be granted forgiveness and readmission to the ecclesial community.
55. This should not cause surprise: to kill a human being, in whom the image of God is present, is a particularly serious sin. Only God is the master of life! Yet from the beginning, faced with the many and often tragic cases which occur in the life of individuals and society, Christian reflection has sought a fuller and deeper understanding of what God's commandment prohibits and prescribes. 43 There are in fact situations in which values proposed by God's Law seem to involve a genuine paradox. This happens for example in the case of legitimate defence, in which the right to protect one's own life and the duty not to harm someone else's life are difficult to reconcile in practice. Certainly, the intrinsic value of life and the duty to love oneself no less than others are the basis of a true right to self-defence. The demanding commandment of love of neighbour, set forth in the Old Testament and confirmed by Jesus, itself presupposes love of oneself as the basis of comparison: "You shall love your neighbour as yourself " (Mk 12:31). Consequently, no one can renounce the right to self-defence out of lack of love for life or for self. This can only be done in virtue of a heroic love which deepens and transfigures the love of self into a radical self-offering, according to the spirit of the Gospel Beatitudes (cf. Mt 5:38-40). The sublime example of this self-offering is the Lord Jesus himself.
Moreover, "legitimate defence can be not only a right but a grave duty for someone responsible for another's life, the common good of the family or of the State".44 Unfortunately it happens that the need to render the aggressor incapable of causing harm sometimes involves taking his life. In this case, the fatal outcome is attributable to the aggressor whose action brought it about, even though he may not be morally responsible because of a lack of the use of reason. 45
56. This is the context in which to place the problem of the death penalty. On this matter there is a growing tendency, both in the Church and in civil society, to demand that it be applied in a very limited way or even that it be abolished completely. The problem must be viewed in the context of a system of penal justice ever more in line with human dignity and thus, in the end, with God's plan for man and society. The primary purpose of the punishment which society inflicts is "to redress the disorder caused by the offence".46 Public authority must redress the violation of personal and social rights by imposing on the offender an adequate punishment for the crime, as a condition for the offender to regain the exercise of his or her freedom. In this way authority also fulfils the purpose of defending public order and ensuring people's safety, while at the same time offering the offender an incentive and help to change his or her behaviour and be rehabilitated. 47
It is clear that, for these purposes to be achieved, the nature and extent of the punishment must be carefully evaluated and decided upon, and ought not go to the extreme of executing the offender except in cases of absolute necessity: in other words, when it would not be possible otherwise to defend society. Today however, as a result of steady improvements in the organization of the penal system, such cases are very rare, if not practically non-existent.
In any event, the principle set forth in the new Catechism of the Catholic Church remains valid: "If bloodless means are sufficient to defend human lives against an aggressor and to protect public order and the safety of persons, public authority must 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".48
The rest of this document is worth reading. Read More

October 13, 2011
By Louie Verrecchio
Catholic News Service recently published a story under the rather dogmatic sounding title, "Dead wrong: Catholics must no longer support capital punishment."
The storyline largely hinges on a condescending quote from Tommaso Di Ruzza, a "desk officer" at the Pontifical Council for Justice and Peace, who said, "It is not a message that is immediately understood — that there is no room for supporting the death penalty in today's world."
No room? Really?
It would seem that either the desk to which Di Ruzza has been appointed came outfitted with a matching cathedra, or... Read More

Avery Cardinal Dulles
Among the major nations of the Western world, the United States is singular in still having the death penalty. After a five-year moratorium, from 1972 to 1977, capital punishment was reinstated in the United States courts. Objections to the practice have come from many quarters, including the American Catholic bishops, who have rather consistently opposed the death penalty. The National Conference of Catholic Bishops in 1980 published a predominantly negative statement on capital punishment, approved by a majority vote of those present though not by the required two-thirds majority of the entire conference.1 Pope John Paul II... Read More
St. Thomas Aquinas
OF VENGEANCE (FOUR ARTICLES)
We must now consider vengeance, under which head there are four points of inquiry:
(1) Whether vengeance is lawful?
(2) Whether it is a special virtue?
(3) Of the manner of taking vengeance;
(4) On whom should vengeance be taken?
Whether vengeance is lawful?
Objection 1: It seems that vengeance is not lawful. For whoever usurps what is God's sins. But vengeance belongs to God, for it is written (Dt. 32:35, Rom. 12:19): "Revenge to Me, and I will repay." Therefore all vengeance is unlawful.
Objection 2: Further, he that takes vengeance on... Read More
By Gregory Koukl
I. The Bible and Capital Punishment
A. Capital punishment was commanded by God in the Old Testament.
1. It preceded the Mosaic Law.
Gen 9:6 Whoever sheds man's blood, by man his blood shall be shed, for in the image of God He made man.
2. It was based on the dignity of man, i.e. man's transcendent value.
Gen 9:6 Whoever sheds man's blood, by man his blood shall be shed, for in the image of God He made man.
3. It was commanded in the Mosaic Law.
a. Twenty-one different offenses called for the death... Read More
Gregory Koukl
There's a reason both the Old and the New Testaments promote capital punishment. That reason was applicable then and still applies today.
Apparently, Jesse Jackson made some comments on "Meet the Press" this morning referring to the possibility of capital punishment for Timothy McVeigh. He said, allegedly, that executing McVeigh would just be a trophy that the people of Oklahoma City would like to get in their trophy case to make them feel better.
Jackson should have been ashamed of his comment. To refer the punishment of a man who is a convicted killer of 168 citizens of... Read More
Gregory Koukl
See how your view of capital punishment says a lot about your view of mankind.
I've been looking for an opportunity to comment on an LA Times letter to the editor from mid-January. I think it's the right time now because of the recent execution of William Bonin, the freeway killer. Though there are hundreds of people on death row, this is only the third execution in California since the renewal of capital punishment. William Bonin was executed by lethal injection, not by the gas chamber. There's a belief that this is more humane. I thought it was... Read More

By: Dennis Prager
FrontPageMagazine.com | Tuesday, November 29, 2005
Those of us who believe in the death penalty for some murders are told by opponents of the death penalty that if the state executes an innocent man, we have blood on our hands.
They are right. I, for one, readily acknowledge that as a proponent of the death penalty, my advocacy could result in the killing of an innocent person.
I have never, however, encountered any opponents of the death penalty who acknowledge that they have the blood of innocent men and women on their hands.
Yet they certainly do.... Read More

By Dennis Prager
Tuesday, July 19, 2005
One should not confuse Jews or Christians with Judeo-Christian values. Many Jews and many Christians, including many sincerely religious ones, take certain positions that are contrary to Judeo-Christian values (which I have defined at length: In a nutshell, they are Old Testament values as mediated by Christians, especially American Christians).
One clear example is the death penalty for murderers. Many Jews and Christians believe that all murderers should be kept alive, that it is not only wrong to take the life of any murderer; it is actually un-Jewish or un-Christian.
Jews opposed to... Read More
By Dennis Prager
By Dennis Prager
By Carrie Severino, September 13, 2011
When the crowd at last week’s Republican candidates debate applauded the execution of hundreds of men in Texas since 2000, it was not the first time I’d heard people celebrate death—recall America’s response to what seemed, at that point, the largely symbolic death of Osama bin Laden. As Christians, we must never revel in death itself; we worship the Lord of Life. But are there cases in which God can use the death penalty to accomplish His plan in the lives of the condemned? Of course.
Many pro-death-penalty activists demonize the murderers, and the... Read More

by Fr. John A. Hardon, S.J.
It is remarkable what appeals to kindness and charity are made to repeal capital punishment. Two thousand years of the Church's defense of capital punishment are brushed aside. Arguments are used to "prove" that capital punishment is inherently evil. No one questions the sincerity of those who claim that capital punishment is not justifiable. But their reasoning needs to be re-examined.
Two Bases for Capital Punishment
According to the critics of capital punishment there are only two possible grounds for capital punishment, namely defense and the restoration of the moral order.
Defense. It is... Read More
Special thanks to the Maryland Catholic Conference for this article. Hat tip: Karen Wingard
UPDATE:The House of Delegates on March 26 approved a plan to severely restrict cases in which the death penalty may be applied in Maryland. Under Senate Bill 279, which Gov. O'Malley has said he will sign into law, capital punishment can only be pursued in cases where there is biological or DNA evidence, a videotape of the crime in progress, or a voluntary, videotaped confession. While this falls short of full repeal, which the Maryland Catholic Conference will continue to work toward, SB 279 is an important step forward and will help ensure that innocent individuals are not executed. Thank you for your continuing support of efforts to uphold the human dignity of all individuals.
Read the rest of the article: http://www.mdcathcon.org/deathpenalty
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."