The triumph of evil over good.
Good has nothing going for it in this movie. The last violent scene felt like the punch line to the whole movie: Not even random bad luck can stop an evil man. In contrast, Moss (Josh Brolin) refuses to abandon or even mess around on his wife and for this he gets greased by a truck full of Mexican drug dealers.
Sheriff Bell (Tommy Lee Jones) is a good guy, but he can’t stop even a single one of the bad things that happens. The old man with the cats saying he would forgive the person who (presumably) had put him in the chair is good, but his situation is pretty bleak. Hell, Moss would have gotten off scot-free if not for his need to do a good deed.
Chigurh (Javier Bardem), the bad guy with the bad hair (and an unpronounceable name), steals the show. The scene with the coin and the old Texas gas station attendant is more effective at articulating an evil character than anything else from recent memory. And it contains no blood.