He's not powerless and he's not subject to the wishes of the state. In OT times, the "state" was chosen by God to worship and represent him in the world. The "divine right of kings" as it was later called meant that the king was on the throne solely through God's action, direct or indirect. The point of the Commandment is that no person has the God's permission to kill a member of God's community. Idolaters were not part of that community, and those sentenced by the community to death would have been judged guilty of breaking one or more Commandments or some other law that the king deemed fubdamental.

God can't be powerless as long as he has the ultimate authority to say what is not permitted. Murder, in this case, isn't permitted, but as a concept, "murder" in OT times only applied within the community.

If a person killed an idolater (a slave perhaps?) presumably the legal system would have determined his punishment, if any. But whatever his punishment, his crime would be more like manslaughter than "murder."

Or so I understand it.

Of course the Commandment doesn't necessarily mean that crimes like manslaughter should go unpunished. All it specifies is that "murder" is absolutely forbidden.

My point is that if killing in war and by legal capital punishment were meant to be forbidden by Commandment, it seems strange from a modern perspective that the Commandment should be worded that narrowly. If they weren't meant to be forbidden, that suggests that the secular morality that finds combat and executions abhorrent is more stringent than divine Commandment.

That idea seems very odd to me. Of course, the eleventh Commandment to love thy neighbor might trump the sixth, but trumping a Commandment sounds odd too. It would suggest that not all biblical guides to behavior are meant for all time, and that human understanding of God can "evolve." But that would be a very big problem for literalists.

By the way, Augustine and other Church fathers were very clear that killing pagan warriors wasn't sinful so long as the Christian soldier did so in the spirit of love. They agreed it was difficult, but far from impossible.