At the beginning of the movie (time 9:15-10:45) Llewellyn (Josh Brolin) checks the cars of the massacre and open the right-hand side door of the truck where an agonizing man asks him for some water (Agua, he says). The window on the driver's side is intact. Llewellyn takes the man's gun and leaves, the door he opened is still open while he walks away.

Later in the movie, when it's night and Llewellyn goes back to the scene to bring some water to the agonizing man, the door is now closed (who closed it?), he opens it again and sees the man that now is dead. The driver's window is broken (time 17:35), like a gunshot broke it. When was it that someone else did it? The drug dealers didn't get to the scene yet. In fact they come at night and find Llewellyn on the spot.