| Matters of life or death cannot be reduced to mathematical quantities.
Per your logic, if ending one life saves ten it's a bargain, eh? Why then end one life when another has already ended? Isn't one death (the murder victim's) enough to sate your bloodlust?
Changing the name from 'murder' to 'execution' is merely semantics. At its root, the question is whether any human authority has the wisdom and the right to end human life.
But don't worry -- the occasional innocent life ended couldn't possibly make a difference to humanity in the long run, eh? | 
