I am open to the argument that some people are just such fucked up psychos that it's better to put them down humanely, than to lock them in a cage until they die on their own, the whole time guarding against the chance that they ever get an opportunity to harm others again.

However, in order to be comfortable actually having any version of that as policy, I'd need to see a justice system that is capable of identifying those extreme cases with 100 percent accuracy, complete fairness to the accused as far as due process goes, and a completely painless and humane method of execution. I'd also like to see one that is aware that the purpose is not punishment, but would be the same sense of tragic necessity as putting down a rabid dog.