If the second platoon would have taken him into custody without incident, then he would have been pacified, and would be a prisoner.
Pacified means unable to offer any resistance. Until he is pacified, he is still a combatant, wounded or not. A wounded man, especially one bent on martyrdom in the first place, is a potential threat until they are rendered unable to offer resistance. And that's the shooter's call. If the Marine felt threatened, he's got a green light to shoot.
Tough luck for the dead jihadi.
When the SAS conducted the rescue mission at the Iranian Embassy in London, none of the hostage takers survived. Some wounded on the ground tried surrendering when an SAS trooper went by, and still got shot. Tough! That's war, especially when the enemy is not a regular combatant (by literal interpretation of Geneva).
I've not been able to follow this story closely. Do you have a couple, or at least one good story that details the fact that the two prone men were not yet prisoners? I'd sure appreciate it. Thanks.