Who is Moss from No Country for Old Men and what is their importance?

Lleweln Moss is hunting antelope when he stumbles upon the carnage that remains of a drug deal gone bad. Several men and even a dog are dead and he goes from vehicle to vehicle, checking the occupants. One man is still alive, and asks him for water but Llewlyn has none. He continues to the back of the vehicle and discovers it full of loose brown powder wrapped in bricks. The reality of his situation hits home, and he immediately regrets ever approaching the vehicles.