The film's plot involves a drug trade gone wrong and its aftermath. Josh Brolin's character, Llewelyn, stumbles upon 2 million and a truckload of heroin amidst half a dozen dead bodies, including a dead dog. He takes the money and plans to retire with his wife with it and move out of their shabby Texas trailer home. But of course other people want the money including Mexican gangs involved with the drug trade and a terrifying loneman psychopathic killer played by Javier Bordem. The plot's simple enough, but what the Coen brothers do with it is nothing short of brilliant.
Bordem's character, Chigurh, is one of the most frightening film creations I have ever seen. What makes him so frightening is not just the blank, cold expression on his face or the way he uses air-pressurized weapons, but his completely nihilistic approach to life. No rules of morality or ethics apply to him whatsoever. His way of rationalizing or accounting for his actions are completely unknowable and chaotic - Woody Harrelson's character makes a comment on it in the film - and thus there is never the option of taking pity, of mercy, or of reasoning. Why he is like that isn't clear, but after a while you start to wonder if he even really cares about the money or if he is simply killing for the sake of killing, according to his own psychopathic reasoning. In one scene he drives upon a bridge and slows down to shoot a bird that is perched on the rail. Why not. Eventually every character seen within the same frame as Chigurh can be assumed dead.
The only exception to this seems to be a flip of the coin, in which Chigurh makes his victim-to-be call a side, giving them that option from either a sense of guilt buried deep, deep within him or just as a way to watch them squirm. I believe the reasoning is a mixture of the two. It is clearly a form of sadism in which Chigurh gets to watch the other person sweat, but psychologically it seems that Chigurh offers fate to be decided not by himself, but by some random chance. The tension between self-determinism and fate seems to play a role in the film's theme; Chigurh is the nihilist for whom nothing matters, the extreme contrast to that being the existentialist premise of self-determined meaning. Chigurh wants to believe in the flip of a coin because he believes life works in the same chaotic, meaningless, random way. This way of thinking allows him to be so cruel in his killing. When Llewelyn's wife refuses to call a side and tells Chigurh that "the coin doesn't decide" but that he does, his eyes widen and we can see the extreme tension in his face: he doesn't want to even allow the possibility that he determines anything, for that would make him accountable for everything.
That may be going a bit too deep, but in any case this film is the darkest, most mature of all the Coen brothers' films. When the credits rolled and I walked out of the theatre, I turned to my friend and said, "That was the most nihilistic film I've ever experienced" to which my friend only looked blankly and shook his head slowly. Hidden in the darkness of the film is some subtle humor displayed through sharp dialogue (a Coen brothers trademark). The usual proportions of a Coen brothers film seem to be a majority of comedy with a dark twist, but this film is the opposite. I believe this is central to the philosophy of the Coen brothers; to them violence and death are almost inseparable from comedy. The absurd nature of violent tendencies within human beings is sometimes so shocking and baffling (think of Steve Buscemi going through a wood chipper in Fargo or Dan Hedaya being buried alive after assumed dead in Blood Simple) that the viewers knee-jerk reaction is a type of confused, half-restrained laughter. There is a very telling piece of dialogue Tommy Lee Jones has in which he describes gruesome deaths from a newspaper article to his young protoge who responds with stifled laughter and an apology to which Jones says something along the lines of, "Sometimes you can't help but laugh at stuff like this."