By isolating the veracity of the charges from what we do next, we then get to the question: Is there such a severe crime that we would never want a given person to EVER appear in open society again?
In other words, is a sentence of "life in prison with no chance of parole, pardon, or commutation?" a valid sentence? Can society ever actually say "We never want to see you again - ever"?
It is my contention that if it is valid to say "This person will NEVER get out of prison" then we have to consider the ultimate punishment - death - as not a punishment, but as a release from a life of living in a degraded status like an animal. Bee, you said in another post that you couldn't even contemplate being in prison. For a person to know they will NEVER get out of prison - ever - I would think is the most incredible punishment possible.
But now let's look at letting the punishment fit the crime. Is this putative "forever in prison" sentence punishment for the perpetrator to match his/her victim? The victim isn't suffering any more. The victim's loved ones are the ones who want the perp to rot in jail forever. To me, the question becomes whether the greater mercy is to put an end to the prisoner's suffering or force him/her to spend a lifetime in a useless situation.
In that case, rehabilitation becomes moot. Punishment is going on, but does life in prison actually fit that crime? Deterrence occurs whichever way you go with this case. And that leads to retribution. But it isn't retribution is a form of tit-for-tat? The victim's experience is not the source of this retribution - it comes from the loved ones who spend the rest of their lives without that loved one.
I think that in cases where the prisoner is going to spend forever in jail, die behind bars, never see family again, and other things like that, then we ought to offer the person the option. Remember, we have isolated "determination of guilt" from subsequent punishment. Tell the prisoner "We know you did it. Confess, then choose whether you want to live forever in a cage or whether you would like to end your suffering." Then honor their answer.