I like life imprisonment for heinous people specifically because it seems like the less merciful option. Look at how many mass shooters and terrorists also take their own lives during the act - suicide is one of their objectives. If we can capture them alive and make them live in a small room, eating unexciting food and sleeping on thin mattresses for decades still to come - that’s the ultimate rebuke to their ideologies of death. Execution, on the other hand, is giving them what they seek.