As I live – declares the Lord Yahweh – I do not take pleasure in the death of the wicked but in the conversion of the wicked who changes his ways and saves his life. Ezekiel 33:11 (source)
I have been prey to the deepest anxiety for fear your Highness might perhaps decree that they [the murderers of a Catholic priest] be sentenced to the utmost penalty of law, by suffering a punishment in proportion to their deeds. Therefore, in this letter, I beg you by the faith which you have in Christ and by the mercy of the same Lord Chirst, not to do this, not to let it be done under any circumstances. … we do not wish that the martyrdom of the servants of God should be avenged by similar suffering, as if by way of retaliation. … We do not object to wicked men being deprived of their freedom to do wrong, but we wish it to go just that far, so that, without losing their life or being maimed in any part of their body, they may be restrained by the law from their mad frenzy, guided into the way of peace and sanity, and assigned to some useful work to replace their criminal activities. It is true, this is called a penalty, who can fail to see that it should be called a benefit rather than a chastisement when violence and cruelty are held in check, but the remedy of repentance is not withheld? Augustine (source)
We know enough to say that this or that major criminal deserves hard labor for life. But we don’t know enough to decree that he be shorn of his future – in other words, of the chance we all have of making amends. Albert Camus (source)
Execution obviously removes any possibility of rehabilitation; rehabilitation both in the sense of restoring someone to the family of humanity after repentance and forgiveness, and in the sense of exoneration, of the undoing of a miscarriage of justice.
More on capital punishment.