D.B.Hart:
Again and again, the New Testament demands of Christians that they exercise limitless forgiveness, no matter how grievous the wrong, even in legal and public settings. And it insists that, for the Christian, mercy always triumphs over judgment. In a very real sense, Christian morality is nothing but the conquest of proportional justice by the disproportion of divine love.
First, his initial claim is simply wrong: mercy is not unconditional, and it is not universally applicable.
Hence Augustine says (De Civ. Dei ix, 5) that “this movement of the mind” (viz. mercy) “obeys the reason, when mercy is vouchsafed in such a way that justice is safeguarded, whether we give to the needy or forgive the repentant.” (Aquinas citing Augustine, ST II-II 30,3)
Both of Catholicism’s greatest theologians recognize that mercy compliments justice. Mercy does not trump justice, and second they both recognize that mercy is not universally appropriate. To receive mercy one must be repentant. Forgiveness is conditional.
In no passage of the Gospel message does forgiveness, or mercy as its source, mean indulgence towards evil, towards scandals, towards injury or insult. In any case, reparation for evil and scandal, compensation for injury, and satisfaction for insult are conditions for forgiveness. (JPII, Dives in Miserecordia)
Second, even if someone is forgiven this does not imply that he escapes punishment.
God, after describing himself as “a God merciful and gracious … forgiving iniquity and transgression and sin”, adds: “yet not without punishing” (JPII, General Audience, 1999)
Third, Hart speaks of the obligations of the individual to forgive and implies that this is also the obligation of the State, but this is decidedly not the case. The individual is obligated to forgive, but the state is obligated to punish. What is forbidden the individual is the duty of the State.
Fourth, if you argue that mercy forbids the state from executing a prisoner you first have to acknowledge that the just punishment for his crime is death, otherwise it would be opposed as unjust, not unmerciful. That, however, leaves you with no objection whatever in those cases where the criminal is unrepentant and therefore does not satisfy the obligation for mercy to be offered. Remember, both Aquinas and Augustine (not to mention JPII) recognized that mercy is conditional and not always appropriate.