There is a lot of confusion and disagreement among people about the use of the death penalty for criminals. Many claim to speak on God's behalf, on both sides of the issue. No one can speak for God.
The Catholic Church is entirely correct and within their rights to take a stance that opposes the death penalty. This is because the Catholic Church is founded on the principle of role modeling themselves after the admonitions and teachings of Jesus Christ. Jesus did not address himself to issues such as the death penalty for criminals. He did, however, convey God's New Covenant that emphasizes the continuing power of redemption, even for those who have committed the most grievous sins. In the sacrament of Confession, for example, the Catholic priest who models himself after Christ must listen to the totality of a person's sins, no matter how dreadful, and apply absolution as appropriate. So while the Catholic Church does not explain it in exactly this way, I'm trying to make it as easy to understand as possible. The Catholic Church fears that executing a criminal shuts down the possibility, no matter how remote, of that person finding personal redemption while still alive and able. This is why the Catholic Church opposes the death penalty. It feels that it is closest to the spirit of what Jesus personified by leaving open ended the opportunity for any person to seek redemption and reconciliation. So this is why I defend the Catholic Church's doctrine in this regard.
However, this does not mean that God has "said" that the death penalty is "wrong." Indeed, both the Old Testament and the Islamic Qur'an record God's permission of the death penalty in specific instances. Therefore one can be in compliance with God's will by being a Catholic who opposes the death penalty AND being a non-Catholic who favors it (if it is done, of course, as rarely, fairly and mercifully as possible). God does not take sides on this issue. Furthermore, one is not more "virtuous" than one's neighbor if you take one position and your neighbor holds the opposite belief.
So I have said that I approve of and endorse the Catholic Church's opposition to the death penalty. I believe their theory of giving a person a chance to repent, confess, and reconcile his or her self to God until the last moment of their natural life is the most Christlike stance to have. Further, I think it gives a lofty but understandable goal of a society where the death penalty is not necessary.
However, let's get realistic for a moment. Cain slew Abel, his own brother, out of jealousy of the goodness of Abel's providing of sacrifice ritual and goods to God. God spared Cain and forbade anyone from harming him. But think about it. Cain did not run around telling every brother to slay his brother as a solution to their jealousy and anger management issues. Cain did not start a business of being a hit man. Cain did not make a movie about the killing and profit from it. In other words, Cain sinned, was forgiven and did not perpetuate his sin to the harming of other innocents or the stability of the society at large. We cannot say that about the many criminals today who merit death row. Society has a very difficult situation where there are criminals who continue to drain and warp society even from prison. I am perfectly alright with societies who in their pious tradition and in their desire to protect the public who continue to have a death penalty. With the prolonging of life and the heinous nature of many crimes today, sometimes the death penalty is a very reasonable way to manage the damage that has been done by the criminal. While I don't openly cheer when the Saudis execute someone, for example, I do not lose one wink of sleep over the criminal getting the well understood and well advertised penalty that he or she merits for their crime in that society.
It is the same here in the United States. As a Catholic I believe in the possibility of redemption up to the end. But as a citizen and a realist I know two things. One is that Catholics cannot even populate their parishes with priests for their law abiding laity, say nothing about minister in the prisons on a daily basis. So who is supposed to "do the work?" The Holy Spirit? Catholics would have much more credibility if they had the entire prison population eligible for the death penalty totally covered with their ceaseless attempts to convert their hearts. The second thing that I know is that there is a ferocity of criminal in this generation who is outside of the reach of normal humanity, say nothing of spiritual appeal. Baby raping murderers come to mind. Remember that Jesus said it is better to cut one's own hand off than harm a child. Jesus would not blink an eye at the fair, just and merciful execution of a baby raping murderer. Jesus would explain that if you read the Old Testament, sometimes the loss of one's life enables an unrepentant egregious sinner to argue for mercy before God. You will read in the Old Testament examples of where God explained that a king who slipped into idolatry will die soon, usually in battle against an enemy that God has raised up against him. This is an example of where the pre-Messiah teaching indicates the possibility that the penalty of one's life on earth can have merit for the unrepentant (or more accurately, those who are too "damaged goods" to achieve full mindful repentance during life). Jesus alludes to this in a way when he says what he did about it being better to cut one's hand off than harm a child, or pluck one's own eye out. He is not just being figurative for the purpose of teaching. He is saying that literally there is merit in physically preventing one's self from doing what is morally evil to another. One could use the example of castration of a chronic sex offender against children as being justifiable completely by what Jesus taught. But that is another topic, that of punishment, and here I speak only of the viewpoints and teachings about the death penalty.
I hope that you find this useful. I'm really tired of each side staking out some sort of moral superiority over the other side, when actually neither side can explain themselves within a theological context with any depth of understanding or accuracy.
Just a personal aside, since people are so fond of creating an entire life and career about misunderstanding me. In college we young people would have these discussions, of course. I would tell people that I'm "a liberal who supports the death penalty." I've later learned that people think "Ah ha! She's a Catholic who opposes Church doctrine! And she's mean!" Nooooo *said with clenched teeth patience*. I support the Catholic stance that in the ideal world that no one should be executed because that is a human defined end point to their chance for repentence and conversion. So I support the Church doctrine. However, I support the state's right to have a penalty system that includes the death penalty for a set of crimes where the death penalty is in the public interest and is fairly and rarely administered. That is, of course, an application of what Jesus said, "Render to Caeser what is Caeser's and to God what is God's." Notice that you did not see Jesus picketing and demonstrating at the weekly crucifixions carried out by the Romans. The civilly imposed death penalty was "not his thing." Saving souls and teaching people to live as Christians "in hope" was his thing. So the Church can correctly take a stance that death penalty terminates the hope of conversion by taking the life of someone who might still take a chance at repentence and conversion. However, Jesus fully supported the civil system that forms the society within which one must live. (This was, in fact, a point of disappointment for some of his followers, that Jesus would not oppose the system where it most grated them). Too bad! Deal with it! Jesus had only two years or so of public ministry and he was there to be the Messiah and bring the New Covenant, not comment on what kind of chariots with what mileage to buy, the civil penalty system of the Romans and the Jews, whether pollution is bad or not, etc. If you were able to "ask Jesus" about any of these modern day issues, Jesus would tell people that this is why they should be well formed in the entirety of their faith (the WHOLE Bible, for example) for guidance as to God's will. He would tell people to perfect their knowledge of the complete teachings of their own faith and let that gradually mature and inform them in their civil responsibilities.
So this is why I support the Catholic Church's hopeful stance opposing the death penalty, but have absolutely no problem living in any state that has a fair penal code that includes the death penalty. If only someone had asked me this a long time ago, instead of just evesdropping, spying, distorting and "judging" what I've said both publically and privately. I have not been inconsistent. Humans are inconsistent in their intelligence and understanding, and have a mean spirited lack of generosity in looking for stumbling blocks rather than illumination about either issues or my overall authority.