I saw the film during a Law of Land Warfare/Geneva Conventions class a while back. I could be wrong, but as I recall he made the face when the gun came level to his head, and held it until he got ventilated a second or two later.
I'm not entirely sure what Ramirez's take is on this, because the VC in question was a combatant operating out of uniform. He had been captured while engaging in combat, and was, IAW said rules of warfare, executed. If I put on an Iraqi uniform a few months back and was captured, I wouldn't merit POW status. Same goes if I took my uniform off and tried to blend in.
So politics, in the case of this cartoon, is implied to be justified in its actions of capping W as an illegal combatant. (Selected not elected? He lied on WMD?)
Take your pick, I don't buy it. The deep meaning of this cartoon only references the superficial meaning of the photograph.