Fortune in mythology relates to the goddess Fortuna, who controlled fate, chance, and good luck. She often teamed with with the goddess Nemesis, the back end of Fortune who checked against greed. Together, the pair brought both prosperity and disaster, often in that order.
In No Country for Old Men, the good luck that Llewelyn finds is swiftly and relentlessly followed up by Anton Chighur, who plays the part of Nemesis. He controls fate with money, the flip of a coin:
What's the most you ever lost on a coin toss?
...You've been putting it up your whole life you just didn't know it. You know what date is on this coin?
...Don't put it in your pocket, sir. Don't put it in your pocket. It's your lucky quarter.
Carla Jean tries to deny him as an agent of fate:
The coin don't have no say. It's just you.
Well, I got here the same way the coin did.
In the end, the coin catches up with everyone: Chighur wins. He is an agent of fate and death, a constant reminder of inevitable suffering for those who come in contact with drugs. Llewelyn has taken blood money, pried it from a dead man's hand. Llewelyn is in denial that his hands are bloody too.
Indeed, fate unmasks Llewelyn--it is also a reckoning. Like the previous editor said, Llewelyn arrogantly underestimates the effects of his own greed and its implications for his family. To Chighur, Llewelyn says:
I don't need to talk to you.
Chighur: I think you do. Do you know where I'm going?
Why would I care where you're going?
I know where you are.
Yeah? Where am I?
You're in the hospital across the river, but that's not where I'm going.
Blood will have blood: Chighur even gets Carla Jean as recompense for Llewelyn's transgressions. Chighur may be a ghost, as Sheriff Bell admits, and he may be injured, but he is still the coin relentlessly pursuing those who relentlessly pursue the big money.
One can argue the main character, Llewelyn Moss, stumbles onto fortune in the form of $2 million with no witnesses to his taking it, and he is under the impression that his life and that of his wife has changed forever for the better.
What finding the money unmasks is the character (or lack of character) of Moss, who cannot let go of his desire to keep the money, his arrogance that he can take care of himself and the people who are after him, and that his personal flaws and inexperience will endanger his family.
Llewelyn, in other words, is still Llewellyn. A poor, relatively uneducated veteran who becomes a rich, relatively uneducated veteran - and the fortune is what ends up killing him.