I have struggled with this question for years. For me, it finally came down to...
What guarantee do we have that the murderer will spend the rest of his life behind bars? What is to stop some future court from having mercy on him and releasing him? What if he escapes? What if the nature of incarceration changes and he ends up spending the rest of his life in what amounts to a luxury resort?
Since there is no guaranteed “life in prison” I say that those who argue for it are either deceitful, deluded or ignorant.
In order to ensure protecting society today and tomorrow, it is society’s responsibility to make absolutely certain that this person never does it again. The only way to be absolutely certain is the death penalty.
You've ID'd the root problem: a broken penal code and prison system that does not put restraint (protection of society form dangerous criminals) as the #1 reason for incarceration.
It's easier to kill 'em than to fix the broken system. Nobody cares about criminals generally which makes it easier to stay with what we got.
For those who disregard Jesus’ own words on capitol punishment...
What guarantee do we have that the one executed today is not found later to be innocent of the crime he or she was executed for, as so many others have been?