Life in solitary is unconstitutional.
How then do we protect other inmates?
Supermax prisons seem to prove that it can be done. But not all prisoners are in supermax prisons, and no system is foolproof. It seems to me that a more likely problem is that a future government will release murderers.
Regardless, the Church acknowledges the right of the State in principle to exact the death penalty. It is up to those in authority to make the determination of whether the death penalty should be applied in a particular case according to the principles described by the Church.