I understand where you are coming from, Mod, but I don't know really how you can have it both ways. We want a system where an infallible interest can look at someone and KNOW, and then deem them worth of death. Imagine all the societies that professed that kind of system, and what kind they really were.
The fallibility of our system is our freedom, the fact that our peers judge us, and that ensures that no one person can sit around and kill whoever they like. That isn't something to worry about, that makes the having the option of the death sentence tolerable.
No, the power we have is in our hands, and the only other option is to say no one should have the power at all. To say that leaves people like bin Laden laying around in jail, writing books and giving interviews, too. A fail-safe system is always insecure. If it wouldn't bother you I might write something along those lines, if you wouldn't think it hijacking your theme.