." They'll live better than a significant portion of your population.
Except they'll have to sleep with one eye open every night to make sure they don't get shived (or butt****ed) by their cellmate. They'll have to be careful so that they don't get boiling water poured over them should they get a job in the kitchen (this is what happened not long ago to child killer Ian Huntley). They'll have to check their food for broken glass. They'll have to watch they don't get attacked by 20 burly guys in the corridor (and that's just the guards).
They are going to be experiencing living hell for decades to come, a much worse punishment than executing them.
Plus, these guys want to be martyred anyway. They want to die rather than be in prison. We should not give them that satisfaction.