But Iona, God didn't command it because God doesn't exist.

I can understand that if a murderer says "I believe in all that crap" then by the murderer's leave, punishing him or her by the bible would be ok for the murderer. But it wouldn't be ok for the rest of the human race, because we don't need imaginary friends or big book of fairy stories to tell us murdering others is not a nice thing to do so don't do it.

Proof if ever you need it that evolution exists. In most western countries, we have evolved to the extent that a majority of people, enough to convince governments in fact, realise capital punishment is a grotesque crime in itself, whereas less advanced countries such as Iran, USA, China etc still have enough superstitious low IQ voters to drown out the rational good people who also live there.

Nothing to do with religion. Religion is given as the excuse for human cruelty, not the reason, and for that, add hypocrisy to the list of why most people dismiss religion as irrelevant nonsense.