"And how do the wise ones of the world know who is unarmed and armed in a split second?"

You don't. That is the very nature of combat, and that is why soldiers train. And they ARE NOT trained to do what that Marine appears to have done, which is to summarily execute a defenseless human being.

That said, we also know that there is a military double standard that says "fuck the rules" and that allows atrocities to occur fairly often, anytime the military is engaged in combat, anywhere in the world. Especially military and paramilitary organizations that know you can usually get away with committing atrocities. Every soldier who engages in committing atrocities knows they might be caught, but that it is highly unlikely that they will be, or if they are caught, that there will be any negative consequences for them.