If this guy was pretending to be dead, (and apparently he was) then it was proper for him to be shot whether he was armed or not.
That was WWII. The Geneva Conventions of 12 August 1949 were after that, partly because of the problems that WWII methods caused. So were Protocols I & II (1977), which pretty clearly state that you can't just shoot someone who is "hors de combat" (Protocol I, Article 41).
Admit it...if:
People can say all you want about the insurgents being unlawful combatants (are they all terrorists? We have evidence there are different groups, not all fighting for same cause) and the Geneva Convention not applying, but I think we have far more to lose if we just toss it all out the window, saying "all bets are off."
We are better than these animals, aren't we?