Couple of quick thoughts on that:
How do you fairly and accurately decide which ones can't be rehabilitated at some point in their lives?
How do you make sure that they are truly guilty? The idea of "safety of others" is counterbalanced by the worry that someone will accidentally or deliberately be wrongfully executed. I mean, it's not like we have a great record of that in this country!
I do understand the frustration with the current system, and it is in bad shape, but I'd be the first out of the country if we moved to a system where we make a quick decision on whether someone is worth rehabilitating or just killing, and hope that that we're right.