I’m not sure there are people so unrecoverable that they need a lifetime in solitary. I’m fact I’m not sure how you pass the cruel and unusual criteria with that. Even in super max prisons for people who WANT to go out and kill strangers for example, they are able to regularly socialize and exercise and have mental stimulation. So no I don’t think there are a lot of people where spending extra money to kill them would be “more humane”. Seems more like a straw man/hypothetical than a practical reality.

Well it always costs more, in the US Justice system, to execute someone than to keep them in prison for life. So that alone throws out the utilitarian approach. We’re all paying extra just to kill him now than if we just kept him locked up for life because he might be a direct threat to everyone and not be rehabilitated.