Punishment (retribution) is only one of a number of reasons to incarcerate someone (the three others normally cited are deterrence, rehabilitation, and incapacitation). I wish we had better metrics for the probability of recidivism. If we knew with a high degree of certainty that a given murder was a one time thing, we could let the perpetrator out very early. On the other hand, if we know that the perpetrator is likely to reoffend, or if we could show that light punishments had less of a deterrent effect, or if society just wouldn't stand for a lesser punishment, we could keep them locked up for longer. Unfortunately we just don't have that level of introspection into the human psyche.