Taking into account the events leading up to the fatal shooting and the unfortunate actions of the young Afghan, if Soldier C had a genuine belief he was dealing with a suicide bomber and that his life and that of his patrol was imminent danger then his actions would be justified under the laws of self defence even though subsequent events proved that there was no such danger.
Oh come off it. They kid isn't wearing a vest, doesn't move to seemingly initiate anything, and he wasn't shot immediately. The operator stands next to him for a while and asks three times whether he should kill him. There's no chance he had a genuine belief he was in imminent danger of being killed by an SIED.
Anyone who looks at that and pretends that it isn't a war crime either isn't being honest or doesn't understand LOAC.