First off, thank you for being able to discuss this rationally, a lot of people are unable.
I think you are confusing "moral" with "justice", but that is splitting hairs. Our definition of justice evolves with society, yes. I am an absolute moralist so I don't believe that what is absolutely right or wrong ever changes, but our understanding of it does. I certainly agree that we should strive towards making our legal system as fair and just as possible, as well as striving to make our punishments as moral and right as possible. We totally agree on that.
I never said you suggested we take jail off the table, I simply used that to show the flaw in your argument. Just because someone has been found to be innocent later of a crime is not a reason to abolish the punishment for that crime. It does not make logical sense whether the punishment specified is a death sentence or a prison sentence.
Other nations and individual states can do as they like, it isn't a factor that is likely to sway my opinion. Let me give you a hypothetical.... now this hypothetical will be as brutal as possible but if you can concede that death is ever a fair punishment then it is simply a matter of drawing the line.
Lets say a man plans out and kidnaps 10 children. He rapes, tortures, mutilates and murders all of them for the sole purpose of his enjoyment. Let us further assume that this is all caught on camera, that the man confesses, and that the man is found to be in full mental facilties (I.E. he knew his actions were wrong and had the ability to not act on them). Would it be more just to kill him or keep him in prison? What if he is severely claustrophobic and the very prison cell would be a constant torture?
Ultimately it is a judgement call and I have heard excellent reasons to abolish the death penalty... but I find there to be overwhelming reasons for it. If a person knowingly and intentionally causes death, torture, ect. with premeditation then I believe that they have abdicated their right to live. I don't expect to convince anyone to change their mind, this is something that people decide in their hearts is right or wrong and only after that do they use reason to buttress their position. I am consistent... I believe in early term abortions, death penalty, and euthanasia.