Criminals have often done terrible things, whether because of "they make bad choices" or "they are unable to make sane choices." In the worst case scenario, these things are incurable, either by medical procedures or by punishment, and their criminality is likely to recur. How do we deal with these broken people? "Humane" prison (comfortable-enough segregation from the general population, coupled with real, universal observation to protect them from each other) seems to me to be the best route, with the death penalty as a distant second.

That has the added benefit of serving people better who are the same but treatable, either via behavioral, psychiatric, or medical interventions. At the very least, it doesn't make things worse for them, which prisons as-they-exist-today currently do.