I'm a little bit (or a lot) old school on this. I think prison is not for rehab but for punishment. I say this as one who has a sibling in the penal system whose job it is to help inmates become educationally equipped to handle the outside world. And I'm fine with that, as long as its in the context of justice and punishment.
Here's why I think it has to be that way: if you focus on rehabilitation or deterrence or even no-longer-a-threat, you're not asking the right question. I think the question has to be this: what did the guilty party deserve?
Focusing on rehab makes the guilty person into a sick person who needs to be cured, not an offender who needs to be punished. Could the person be ill, addicted, infested, demonic, or anything else? Yes. And should some sway be given to a judge in that moment? Probably. But focusing on rehab instead of punishment takes the innate "ought" out of the equation. It also victimizes the offender. I call that no good.
Focusing on punishment as a deterrent to others misses the point altogether. It leaves out both victim and perpetrator and focuses on the people who aren't even involved in the matter at hand. Blech.
Focusing on no-longer-a-threat makes the guilty person into a risk probability who can be measured, weighed, or evaluated. But again, the question of what they deserved is not being asked. Instead, it's a simple utilitarian, mathematical equation. If the risk is over X-percent, he stays in jail. If not, let him out. I call that no good too.
C.S. Lewis said this and it couldn't be said any better...
Mercy, detached from Justice, grows unmerciful. That is the important paradox. As there are plants which will flourish only in mountain soil, so it appears that Mercy will flower only when it grows in the crannies of the rock of Justice; transplanted to the marshlands of mere Humanitarianism, it becomes a man-eating weed. (from the essay, The Humanitarian Theory of Punishment)Justice should be given in instances like this. And a pathetic 21 years for 77 murders falls woefully short.
And then I remember that I have stood before God as guilty as Anders Breivik. And infinitely more so. And while public justice should be unswerving, God's justice was accomplished and His mercy was shown at the cross. It's enough to devastate.
But that's just me thinking thoughts...