I dont think there are that many convicts coming out with MBA's
Just the reduction in recidivism alone pays for all the education that a rehabilitated convict gets.
This seems to be a fundamental difference in left and right leaning people. The further right you go, the more people want to punish people, the further left, the more people want to rehabilitate people.
Consider for yourself, is it easier to show you in a well reasoned argument what it is you have done wrong and shown a better way that avoids those problems, or to hurt you enough that you are afraid to do it again?
Some people can't be rehabilitated. I would say that serial killers, pedophiles fit into this group. I mean, could I change your sexual orientation by putting you in jail for a while and threatening you? I wouldn't want them tortured or harmed, though vengeance seems such an easy position to take. Rather, I would want them to be removed from society for good, and placed where they can have some sort of meaningful life away from those they would harm if they could.
In this way, they are easier to manage which makes those who work with them on a day to day basis, job easier and less of a chance that they will risk it all to escape. Removing all the temptation helps to keep them out of trouble when they can't resist their urges themselves.
Killing someone doesn't allow for mistakes. Using Milgard, Marshall and others as examples, show that mistakes can be made and drawn out for more years than it takes to process someone for the needle. I would rather err on the side of safety and lock them up for ever than to kill an innocent person.