The only problem with saying that it's ok if we have irrefutable proof [which I do agree with, btw] is that it's debatable whether there is ever such a thing as irrefutable proof. I mean, in all cases where people have been sentenced to death there has obviously been what was, at trial, considered to be 'irrefutable proof' of their guilt, or at the very least the balance of evidence was strongly tipped in favour of their guilt. And if some of these decisions have since been proven erroneous by advances in medical science/technology then I don't think it is unreasonable to suggest that we are far from having full-proof methods of verifying a person's guilt, and that it is therefore irresponsible to sentence someone to death who could quite easily be proven innocent a year after their execution.