Thou shalt not kill

nobody takes "thou shalt not kill" seriously.  people kill germs, flies, spiders, cats and dogs, deer, cows, sheep, chimpanzees, elephants, whales, and other people all the time.

what people understand the adjuration to mean is Thou shalt not murder.

nobody takes "thou shalt not murder" seriously.  people murder other people all the time, for all kinds of reasons.  they even murder themselves.

what they really think it means is "Thou shalt not murder those I have marked for protection."

According to the Book of Mysteries, the only one who was ever marked by the Ineffable Creator for protection was the First Murderer.  Not even the Man born of His Infinite Grace into a life of suffering did He so mark.

I think that is a fairly strong case against the death penalty, whether executed in a prison or on the battlefield.