There was far, far more than enough cause to believe he, and anyone else in that compound, may have been booby-trapped. In a combat situation, this was the perfect way to deal with that threat.
Remember that marine that shot a wounded enemy combatant a while back, and was tried for murder? His entirely legitimate defense was that he wasn't sure the man wasn't booby trapped. This, after numerous squads had been killed by combatants they had wounded pulled pins on closely-held grenades.
I realize I'm not articulating my argument properly, so I will close with this: That was a dangerous situation to be in. "Conspicuously surrendered" is one thing, but in a combat situation, how are you to be sure that the man who is surrendering, isn't packing explosives in his underpants? You aren't, until he's either dead, or it's too late to do anything about.