I’m not sure how you read into my post the idea that I think some killing is “copacetic.” (Now there’s a word you don’t often run into any more.")
I don’t think that it makes any difference whether or not the commandment originally used the word now translated as “murder” or the word now translated as “kill.”
The commandment merely lays down a ban without specifying any punishment. How various killings are to be treated is spelled out in various other places. Exodus, Leviticus and Deuteronomy for example. It is analagous to Congress passing a law establishing a new agency and leaving the details of its actual administration to be filled in by the Executive Department in a series of regulations.
As far as the question in the OP, it seems to me one prime message of the Old Testament is that that God’s directions are to be obeyed. If He says kill everyone and everything - men, women, children, livestock, crops - then that is what is required on penalty of offending God. Or if He says to kill all the men and keep the "women and the little ones and the cattle [and ] … all the spoil thereof to yourself as in Deut. 20:13-14 then you do that.
So it would appear that the devout Christian could easily engage in war as a combatant if the war is presented as upholding God’s will. Even Hitler claimed that he was fulfilling God’s will. In Mein Kampf, Book 1, Chap. II . he wrote, “And so I believe today that my conduct is in accordance with the will of the Almighty Creator. In standing guard against the Jew I am defending the handiwork of the Lord.”
Now, I don’t say that a Christian in the military in WWII, or Korea, or Vietnam is in any way comparable to Hitler. I merely say that rationalizing is easy and if the enemies can be made into Amalekites by propaganda then killing them can easily become the Lord’s will. They can all be killed to the last person and their cities destroyed and their land strewn with salt so that they will not be a problem in the future.
In WWII US Secretary of the Treasury Henry Mogenthauargued for just such a solution for the “German Problem.” In my WWII bomb group we regularly bombed cities that held women, children, old people and war wounded. The mission planners knew it and so did we, including the many Christians among us. As I said, rationalizing is easy. For one thing, if you were ordered to go and refused you could easily go to a military prison, or worse, and someone else would be sent to do the job. In other words, the rationale is that the job will be done and your opposition won’t change that, it will merely get you in deep trouble. Business executives find this rationale very useful.
I do not claim that these are the particular rationalizations that are used by Christians or were used by those of us in the bomb group. In our case that matter never came up as far as I know. At least it wasn’t a subject for great discussions among us. I am just pointing out in answer to the OP question some of the ways in which a Christian can rationalize participating in a war as a combatant.