I rarely post... However!
No matter what way you look at what happened with this Marine, he walked up to an injured fighter and shot him. Prior to doing this he asked if the helmet cam was switched off (because he knew what he was about to do was wrong), then stated he was about to breach the geneva convention and finally muttered some Shakespeare before killing the wounded enemy fighter! After the deed was carried out he then tells his section that it didn't happen.
I am no head doctor, but that does not sound like a man on the edge who had PTSD or "diminished responsibility". He killed someone in cold blood and made a conscious effort to do this despite several comments that demonstrated he knew the legality, morality and gravity of his actions and subsequent attempts to cover it up.
That being said, I do believe there are extenuating circumstances such as being battle weary, under-manned, numerous contacts and his friends injured. None of these can justify the killing. He was a SNCO who has a duty under service and international law, namely the Law of Armed Conflict, to provide medical assistance to injured enemy combatants.
This is not something new and I am 100% confident he would have been taught this on several occasions.