At least for murder, one of the strongest predictors of whether a person will kill is if they've killed before. So prison is productive because it makes it more difficult (though not impossible) for a person to keep killing.
Some people are or became irredeemably antisocial and need to be kept away from society at large. Watch any liveleak video around the mexican cartel if you need a reminder of what evils people are truly capable of.
That being said, sure want to properly reintegrate everyone possible. But society understandably does not exactly welcome past offenders back with welcome arms. It is kind of hard to be productive again when few places will even consider hiring you. That isn't a stigma you can just wish away.
Or, do we really think it was down to just this one person's actions?
Really?! Interesting...
I agree, a functioning society needs good law and order, and some way to prosecute and bring criminals to justice.
But I am personally not convinced that sending someone "to jail for a long, long time" is the solution. How is that productive? I think we need to come up with better ways to serve justice than physically restricting otherwise smart or productive individuals to a room for decades on end. As soon as we are done with abolishing the death penalty, I would want us to start looking at abolishing this "decades long locking up" of folks. I don't have a good solution as I said, but I am convinced that this is not it.