Under UCMJ he should walk; it was a war zone, the insurgents were guarding a weapons cache, when confronted they fled. Their affiliation and intent was apparent. I really don't see where the "send a message" statement has any bearing unless someone believes it indicates prejudicial or pre-meditated action. I think it was likely done at that moment as in "hey, why don't I just finish the clip to 'send a message' to other insurgents".
One question is what authority the "interim government" had at the time and what the UCMJ's role therein would be. I believe the US military at the time had (and still has) the authority to pursue and engage hostiles. 3 rounds or 30 makes little difference. Dead is dead. That should be the action in question.
I don't think premeditation will trump the circumstance. Imo the shootings appear justified.
Title 10 of the United States Code is the basis for the existence of the UCMJ. Sometimes people forget that the military is not a separate part of the Government.
Your last statement "Imo the shootings appear justified" is intellectually honest because you are qualifying your statement as your personal opinion and not adjudicated fact.
The comprehensive Article 32 investigation has not yet been concluded. It may or may not provide facts of which none of us are yet aware.