A vital point here is that justice gives the appropriate authority the right to punish offenders up to some limit, but one is not necessarily and invariably obliged to punish to the limit of justice. Similarly, if I lend a man money I have a right, in justice, to have it returned; but if I choose not to take it back I have not done anything unjust. I cannot claim more than is owed to me but I am free to claim less, or even to claim nothing. For a variety of reasons (amongst them the hope of reforming the criminal) the appropriate authority may choose to punish a man less than it is entitled to, but it is never just to punish a man more than he deserves. It is a mistake to argue—as Ewing, for example, does in Chapter II of The Morality of Punishment—that, on the Retributive theory, to punish a man less than the exact amount due is an injustice similar to punishing an innocent man. The Retributive theory is not, therefore, incompatible with mercy. Quite the reverse is the case—it is only the Retributive idea that makes mercy possible, because to be merciful is to let someone off all or part of a penalty which he is recognized as having deserved.
(K. G. Armstrong, "The Retributivist Hits Back," Mind, n.s., 70 [October 1961]: 471-90, at 487 [italics in original])
Note from KBJ: Armstrong's analogy is inapt. If I lend you money, I am indeed free to claim less than what you owe me. But judges are not in this position. Only victims are in this position. A judge is obliged to punish to the limit of justice.
Keith Burgess-Jackson, J.D., Ph.D. 