Spyro:
In response to your points above:
(1) I don't see how an execution is more expensive than maintaining a criminal in prison, sometimes for decades. Even if it were more expensive to execute than to maintain a prisoner, the issue is not money but justice. The real question concerns what the criminal deserves rather than how much it will cost. No nationis well advised to put money above justice. In moral questions like this one, it seems that justice is properly fundamental.
2). I think you are absolutely correct that innocent people wil be executed, and if justice is paramount, as I think it ought to be, then this is a serious failure. No nation's penal system is perfect. And if it is not perfect, then serious mistakes of this sort will happen. If a prisoner is incarcerated unjustly, he can be released and in some way compensated for the injustice. But if he is dead, nothing can be done to right the wrong. On the other hand, if murderers are not executed, they might well murder again, whether in the nation at large, if released, or in prison, if still a prisoner. In that case, innocent persons are still dead, and that does not serve the cause of justice.
3. The issue is not killing but murder. That is, while all murder is killing, not all killing is murder. Murder is unjustified killing. If someone has committed a capital offense and deserves to die, that is not the same as executing a murderer. In other words, your point 3 equivocates on the word "kill" in much the same way I would be equivocating on the word "push" if I said that a man who pushes an elderly woman into the path of a car and a man who pushes an elderly woman out of th epath of a car are the same becuase they both push elderly women.
Thanks for starting a very important and interesting discussion.