But I think it is one adequate way of redressing any criminal's preference for his own interests. And it may be that the Pope is saying, where indefinite imprisonment without substantial continuing danger to others is possible do not put criminals to death. There are difficulties with the alternative reading of GL, the reading which would limit executions only to those criminals who remain incorrigibly dangerous. The Pope does not say. It is always possible that a prisoner will escape custody and do harm, or in anger kill someone inside the prison.
Complete isolation is impossible.
Even prisoners in solitary confinement need to be fed and bathed. They are entitled to basic medical care, if not to limited social interaction with other prisoners and, possibly, with family. The prisoner may ask for a priest, minister or other pastor. Still all this is obvious, that a violent felon continues to have opportunities to be an aggressor must not be sufficient, in the Pope's mind, to justify executing him. I do not doubt there are many such persons.