Beneath Llewelyn Moss' grizzled exterior is a relatively extraordinary degree of compassion, enough so that it at least causes him to dangerously risk his life, and possibly seal his eventual fate, early in the events of the book.
While hunting, Moss comes across the aftermath of a drug deal gone south, finding both the drugs and $2.4 million in cash. Unable to resist the temptation, he takes the money back to his home. While he was at the scene of the crime, he encountered a man who was still alive but unable to move. Moss had told him that he did not have any water to give him.
Moss likely feels guilty about leaving the man out in the desert in extraordinarily pitiable circumstances, and this is no doubt why he decides to leave his house and return to the scene of the crime with a jug of water for the man. Unfortunately, the man who Moss intended to help is already dead at the scene, and the cartel members who killed him are still there. This leads to the game of cat and mouse that comprises most of the narrative.
Furthermore, it establishes Moss as a man who, while flawed, is ultimately good. While he functions within the narrative as a man caught in the middle of the long standing war between good and evil, he is ultimately good enough to put up a decent fight against the unstoppable evil that is Anton Chigurh.