I think we honestly have it backwards in most modern legal systems. Someone who’s done something horrific but has the psychological and mental presence to be made feel remorse for it should be kept alive in prison as long as possible to give them the opportunity to repent of their evil actions and to atone for it - possibly even by volunteering to give up vital organs for transplant.

Those who are incapable of remorse, however, and will never be even capable of repenting and atoning for their sin, are incomplete moral agents, and I think in those circumstances, like an animal gone bad, we must ascertain if the risks of future violence are high enough that a humane execution, such as firing squad or voluntary donation of vital organs, is truly the only feasible way to protect society at large from them.