That's the thing though, I know exactly what you mean but how do you write that down in a way that ensures that it's only used in those circumstances. The only alternative I can think of is leaving it up to the judges, which would get around the issue if the definition but would probably result in public pressure to use it in circumstances where someone is convicted of a crime which causes a lot of emotional response from the general public. I'm with you in that there are definitely cases where it's a waste of time to keep them alive, but I thing the implementation is impossible.