Execution of murderers does prevent any particular killer from killing again, thus protecting the innocent. Finally, the death penalty is distinctive in that it communicates an abandonment of rehabilitation. The one who has taken the life of another is punished by having his freedom taken away for the rest of his life. The deterrent value of life imprisonment is hard to measure, though it is certainly no appetizing prospect. Life imprisonment without parole and under maximum security protects the innocent from being harmed.
Rehabilitation is the big difference.
Life imprisonment makes possible, though it does not guarantee, the reclamation of a soul. Let us assume for argument's sake that deterrence and protection of the innocent come out roughly even with either penalty. Let us likewise assume that the death penalty delivers retribution somewhat more strongly than does life imprisonment, whereas life imprisonment offers the possibility of rehabilitation while the death penalty does not. Holding open the possibility of rehabilitation in the context of life imprisonment fits with my own vision both of God and of a good society. The murderer receives retribution.