Is it really used as a warning though? I presume it’s more like a statement of removing a constant danger from the population without keeping them permanently alone. To your point, if every person has value then you can’t just lock up inveterate murderers with other prisoners and put those prisoners in potential mortal danger.
I’m not sure this is how the death penalty is used in practice since the appeals process is so long, but I’d be hard pressed to think of a punishment worse than perpetual solitude in a small concrete box until you naturally expire decades later with no hope of ever leaving.