It is a well-established (though by no means universally accepted) premise of human behavior that negative reinforcement (punishment) is almost entirely ineffective past the age of reason (about four to seven years old) and of limited value even before then. Nevertheless, most people far older than that continue to desire retribution for an injustice. There are valid and good reasons for this desire, but that it can be a deterrent (a prospective negative reinforcement, in other words) is not really one of them.

As for rehabilitation, it, too, has a meaningful basis in the arena of criminal sentencing, but coercion nullifies the possibility of rehabilitation: a human cannot be forced to rehabilitate, they can only be convinced to obey. Also, the practice of providing heightened opportunities for rehabilitation to criminals contradicts the demands of justice, by essentially rewarding convicts for previous bad behavior and (if this can even be considered separate) effectively substituting advantage for retribution, in a social sense.

For imprisonment to be just and moral, it should be seen as simply sequestration, removing an individual from society. Whether it is a punishment or is an adequate opportunity for voluntary rehabilitation is for the convict to determine. Making their living conditions as sparse as humanely possible based on the nature of their crime is appropriate, but willfully (including through malicious neglect) making their circumstances intolerable or horrifying in a misguided effort to maximize either punishment or deterrence is not merely inhumane, it is counter-productive.

It really doesn't matter how severe or horrendous the crime was. The optimim process from a social perspective is whatever has the greatest statistical likelihood of resulting in a convict both recognizing and accepting the immorality of their actions and voluntarily choosing to rehabilitate themselves. And what we are currently doing in the United States is not that. A large part of the reason we have such a significant problem with crime in the US is because of the prison system in the US, although the teleologies are so complex that most people aren't interested in understanding how that is possible. And the people who most definitely (and erroneously) believe in punishment have any easy time convincing anyone who has any desire for retribution against lawbreakers to vote for the Wrong Wing Party, and spewing atrocious lies about anyone who does manage to understand that 'the beatings will continue until moral improves' is too much of a joke to even be funny.

"My parents whipped the hell out of me, and I'm a better person for it," is something that only comes out of the mouth of a severely emotionally damaged individual.