The Marine did the absolute right thing.
For all he knew the enemy could have a gun, grenade etc etc.
It's war people, get use to it.......
Remember the first guy to die fighting terrorism in Afghanistan after 9/11? It was Johnny "Mike" Spann. (think I got name right - corrections appreciated).
He died because a wounded Taliban detainees grabbed a grenade off a Northern Alliance soldier and blew himself and Spann up. The Taliban were then able to retake to fort and a 10 day long battle ensued with many good guys killed.
In war - if you are in pitched battle, and there is not a way to neutralize a wounded enemy, you have to terminate with extreme prejudice to protect yourself and your fellow soldiers.