From a Christian point of view, capital punishment is evil because it transgresses Jesus' Law of Love and usurps God's prerogative.
In his book Entrez dans l'espérance, John Paul II writes:
There is not, for man, any right more fundamental than the right to life. Even so, a particular modern theory has wanted to deny him this right, going so far as to consider it too "bothersome" to defend. But no other right is so intimately bound to the very existence of the person. The right to life implies first the right to be born, and then the right to live until a natural death: "As long as I live, I have the right to live".
In Veritatis splendor he writes:
Reason attests that there are objects of the human act which are by their nature "incapable of being ordered" to God, because they radically contradict the good of the person made in his image. These are the acts which, in the Church's moral tradition, have been termed "intrinsically evil" ("intrinsece malum"): they are such "always and per se," in other words, on account of their very object, and quite apart from the ulterior intentions of the one acting and the circumstances. Consequently, without in the least denying the influence on morality exercised by circumstances and especially by intentions, the Church teaches that "there exist acts which "per se" and in themselves, independently of circumstances, are always seriously wrong by reason of their object". The Second Vatican Council itself, in discussing the respect due to the human person, gives a number of examples of such acts: "Whatever is hostile to life itself, such as any kind of homicide, genocide, abortion, euthanasia and voluntary suicide; whatever violates the integrity of the human person, such as mutilation, physical and mental torture and attempts to coerce the spirit; whatever is offensive to human dignity, such as subhuman living conditions, arbitrary imprisonment, deportation, slavery, prostitution and trafficking in women and children; degrading conditions of work which treat labourers as mere instruments of profit, and not as free responsible persons: all these and the like are a disgrace, and so long as they infect human civilisation they contaminate those who inflict them more than those who suffer injustice, and they are a negation of the honour due to the Creator".
"Homicide" means literally "killing a human". Capital punishment is homicide.
In Evangelium vitae :
28. ...We are facing an enormous and dramatic clash between good and evil, death and life, the "culture of death" and the "culture of life". We find ourselves not only "faced with" but necessarily "in the midst of" this conflict: we are all involved and we all share in it, with the inescapable responsibility of choosing to be unconditionally pro-life.
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". 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.
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.
On the one hand, John Paul II tells us we have "the inescapable responsibility of choosing to be unconditionally pro-life," while on the other he speaks of "cases of absolute necessity" to apply the death penalty.
The problem with absolutes is that they're absolutely true in all situations. The Pope seems to have missed that. He seems to be offering us relative morality here.
The hierarchy of the Catholic Church now stands in opposition to the death penalty almost as unequivically as it stands in opposition to abortion. While this is a heartening development, lay Catholics - especially in the United States - do not yet seem to have "got the message".
The Church - the whole Christian Church - must clearly teach that the death penalty - for any crime - is intrinsically evil.
Finally (and in anticipation of the arguments of those who would apply Old Testament theology), Jesus gave us a "direct commandment" - by example - when He did not stone the woman and said, instead, that the one without sin should cast the first stone. He was without sin, and He didn't cast the first stone...
When a man kills another man, he may be denying him time for repentance.
He also said (though on another subject) that we are to render to Caesar what is his, and to God what is His. Life belongs to God.
The Law of Love - with His added exhortation to love even our enemies and do good to those who hate us - clearly obliges us not to kill.
Christ Himself was executed legally.