In practice, then, a delicate balance between justice and mercy must be maintained. The State’s primary responsibility is for justice, although it may at times temper justice with mercy. The Church rather represents the mercy of God. Showing forth the divine forgiveness that comes from Jesus Christ, the Church is deliberately indulgent toward offenders, but it too must on occasion impose penalties. The Code of Canon Law contains an entire book devoted to crime and punishment. It would be clearly inappropriate for the Church, as a spiritual society, to execute criminals, but the State is a different type of society. It cannot be expected to act as a Church. In a predominantly Christian society, however, the State should be encouraged to lean toward mercy provided that it does not thereby violate the demands of justice.
It is sometimes asked whether a judge or executioner can impose or carry out the death penalty with love. It seems to me quite obvious that such officeholders can carry out their duty without hatred for the criminal, but rather with love, respect, and compassion. In enforcing the law, they may take comfort in believing that death is not the final evil; they may pray and hope that the convict will attain eternal life with God.
The four objections are therefore of different weight. The first of them, dealing with miscarriages of justice, is relatively strong; the second and third, dealing with vindictiveness and with the consistent ethic of life, have some probable force. The fourth objection, dealing with forgiveness, is relatively weak. But taken together, the four may suffice to tip the scale against the use of the death penalty.
Avery Cardinal Dulles, “Catholicism & Capital Punishment” (2001)