The most common translation of Ex. 20:13 is “You shall not commit murder.” Nothing in there about killing in self-defense or combat.
The Marine in question is confused. If he had any moral qualms about killing an enemy combatant, perhaps he should have remained a civilian.
Combat killing could easily be murder. It wasn’t for a long time after the decalogue that Just War Theory was developed. I agree, this person shouldn’t have signed up if he truly believes all killing is wrong.
The only thing about combat, is that the individuals that are in combat are not the one’s that decide there will be combat. If you leader is evil, does that make your combat ok?
You are absolutely correct. In addition, the Taliban and Al Qaeda have no moral dilemma with hiding behind and killing innocent women and children and killing American civilians that they capture—and theey would have had no moral compunctions about killing him if they had captured him.