In their efforts to hide their theft, they commit more crimes. On top of this, the three men start feuding. The money creates paranoia and distrust, with each watching his back and becoming more and more defensive. When an FBI agent comes looking for the plane and the cash, all the distrust and greed blow up, and few people are left alive. Three seemingly honest and decent fellows lose their decency and suspect that everyone else has as well when some cash is dangled before them.
People are a bunch of greedy fucking assholes.
It's predictable that the problems of the crooks just keep getting worse. If they didn't the movie would only be fifteen minutes long. What is so great is that the downward spiral makes sense. They do it because they have to in order to keep their money. The climax is just about the best I've seen this year.