I haven't seen any reports on how he was killed, and this is conjecture on my part...but given the nature of the enemy, I doubt he was killed by a bullet.
BURN THE GENEVEA CONENTION! IT WAS NEVER MEANT FOR AMERICANS, IT WAS WRITTEN TO PROTECT OUR ENEMIES ONLY!
The reports I have seen seem to indicate that we have some sort of video of this soldier being captured and killed. However the reports are not saying how he was killed.
I would be willing to bet big money that it was not a bullet to the head but more along the lines of the Daniel Pearl execution.
The body has been recovered. Dead of a bullet wound.
Bad conjecture - he was murdered by a single bullet to the head.