Most questions I see asked about the death penalty seem to center on whether it is wrong because of the harm it does to the person who is executed.
What about the harm done to others by keeping a dangerous sociopath alive? Let's posit that we have a person who is so depraved that a prison sentence is no deterrence; and this person will gleefully cause pain, suffering, even death to prison guards and other inmates whenever he has a chance.
Is it reasonable for all these other people to have to be exposed to such danger?
Granted this scenario is an extreme case, that prison guards (let alone other prisoners) never anticipated such a danger to themselves when they first signed up for the job.