"Just deserts" is offender-focused: What ought this person to suffer? The extreme version of this is Kant's, who says that the offender is owed punishment and that it would be unjust to the offender to forgo it. ...
Retribution, by contrast, is victim-focused. Retributive punishment is a public demonstration that what was done to the victim was wrong, and that the victim was worth avenging. ...
One function of public justice is to displace private vengeance, which is by its nature capricious, likely to misfire, and capable of producing a Hatfield-and-McCoy spiral. But if public justice is to displace private vengeance, then public justice must be done. If Pinochet's victims and their families are to be asked to refrain from hiring private thugs to smash his kneecaps, then the public owes it to them to handle Pinochet in court, no matter how old and harmless he now is.
While I am highly sympathetic to the moral claims of victims of violence, I think there are several problems with overly victim-focused justice (and it's a problem that manifests itself in such popular practices as victim impact statements at sentencing, or prosecutorial speeches describing the victim's wonderful qualities).
1. Focusing on the "worth" of the victim may lead to grading crimes on the victim's social standing, popularity, etc. Should the murder of a beloved husband and father, and a valued member of his community, be punished more severely than the murder of a friendless middle-aged bachelor? Unfortunately, a lot of studies show that the "merit" of the victim has an impact on the sentence given to a hypothetical offender, but I think that the criminal's intent still plays a key part in determining appropriate punishment. Let's take two hypothetical situations:
(a) A serial child molester is released after serving a ridiculously short sentence. A few days later, he is accosted and shot dead by the distraught parent of one of his victims.
(b) A serial child molester is released after serving a ridiculously short sentence. A few days later, he is accosted and shot dead by a mugger.
Without condoning vigilantism, I think most of us would give the distraught parent a lower sentence than we would the mugger, even though the victim -- and the end result -- is the same.
2. Victim-focused justice, strictly applied, would make no allowances for intent. If someone forgets about a lit stove and starts a kitchen fire that kills six people, the victims are just as dead as they would be if the fire had been started by an arsonist (and the victims' relatives may be just as angry). But surely no one would advocate the same punishment for the negligent cook and the arsonist. Furthermore, in some cases we decide that the offender should be given a very mild punishment or none at all because his/her mental capacity makes him/her unable to fully understand the nature of the act.
3. Victim-focused justice would make the degree of punishment contingent on how forgiving the victim is (or how forgiving his or her relatives are). If a victim chooses to forgive her rapist -- because of, say, religious beliefs, or the ideoogical conviction that criminals are really oppressed victims of society -- it should have no impact on the rapist's punishment.
The shift from private vengeance to public retribution means more than simply delegating vengeance to the state because it's more practical. It also means a shift to the concept that the crime is an offense not just against the victim, but against society, or against the moral order. Walter Berns and other advocates of retributive justice argue that a crime disrupts the moral balance of the universe, which is then restored by punishment. This may seem like an overly abstract and metaphysical concept, but I think that such a notion of crime and punishment as moral calculus underlies a lot of our thinking on the subject.
Finally, Kant's notion that it would be unfair to the offender not to punish him may seem quaint, but I will point out that in some cases, criminals who were not apprehended have turned themselves in because they were tormented by remorse and felt that being punished would salve their conscience.