A couple of very interesting and well balanced posts from Rapaire and Wolfgang (as usual). I am not really in agreement though with the conclusions you have drawn.
First of all Wolfgang, I think the, "One's too many" argument is very convincing indeed. The state's judgement has to be based on an absolute conviction (as far as possible) that the sentenced man deserves the punishment. We are talking about the state's responsibility - and not that of a revenge killer in a seventies vigilante movie.
Oddly enough, the argument that life imprisonment is worse than execution - which I personally agree with - does not necessarily make it a better deterrent. I would expect most convicted murderers to react emotionally rather than rationally.
The debate on this thread so far seems to have focused very much on the worst case scenario - in reality a very small minority of all murderers.
That brings me to Rapaire's point, which is that if the guilt in a horrendous crime is beyond all doubt, does there have to be a case for execution?
I'll come out with the NOs on that one. A murderer who leaves so much evidence around that his conviction cannot be in any doubt is in fact a bloody fool. However, a truly devious, calculating murderer sets out to leave very little evidence indeed. Who is more deserving of punishment? Who is more likely to receive it? I am against the death penalty because I believe it undermines the justice system. I have very little sympathy with most executed criminals, but that is irrelevant. It's not a beauty contest. My feelings are best left out of it - unless we want to have a Sharia style of justice.