Why go for for a situation like that?
Marine A apparently was in the wrong because the person he shot was no threat to him, and so shooting him is murder......or something like that.
So, how about a fictional Apache pilot who is above the effective range of an insurgents weapon in a country that could be Afghanistan. The insurgent is no threat at all to him, but he engages the aircrafts cannon. After round one the apache pilot will not be able to see the insurgent due to dust kick up. The insurgent may be hit and wounded, and may have dropped any weapon he had, but the cannon shells are still landing all around this unarmed and injured man. If a later round hits and kills the insurgent does that make the apache pilot (long range) or gunner (close in) guilty of murder?
The ROE's involved dictate whether its murder I suppose, or whether it's just killing an enemy combatant.
Edited to add: our current ROE's don't work and can't work IMHO. A squaddy seeing a baddy who has just beheaded, or blown up his best mate can't shoot the baddy once any weapon has been laid down. The same goes for an insurgent sniper, pop down your weapon and raise your dish dash and you are free to go. But if the sniper is behind a wall having a brew after killing a Brit he may be on the receiving end of a javelin or hellfire, even though he is unarmed.
I sometimes wonder if our ROE's might be a bit more warlike if they were written by the Taliban.