Braddock Washington is the embodiment of the values critiqued in Fitzgerald's satire: an insatiable desire for wealth, the absence of religion, and the will to destroy others for personal gain. Because he descends from George Washington and Lord Baltimore, it's clear that Braddock is in many ways a symbol for America's own founding and expansion into the West (see "Symbols, Imagery, Allegory" for more on this parallel). Given this connection, Braddock's lines of dialogue take in a particular importance. Consider the following two statements:
"How could a man of my position be fair-minded toward [the imprisoned aviators]? You might as well speak of a Spaniard being fair-minded toward a piece of steak." (6.37)
"Cruelty doesn't exist where self-preservation is involved." (6.44)
This is just the attitude that Fitzgerald critiques with his story. Braddock not only is willing to exploit others for his own purposes, but he also thinks there's nothing wrong with it. He thinks it's "perfectly natural," as Kismine will later say, to get as much as you can out of other people (8.33). This is the attitude, the narrative seems to argue, that led to things like slavery.
Of course, the irony in all of this is that Braddock is himself a prisoner to his own wealth and in his own château. Sure, he has the aviators imprisoned below the ground, but he has himself imprisoned in a much larger, much better disguised prison. He is, in many ways, a slave to his own obsession with wealth. His entire life functions around hiding the diamond from others. He serves this obsession flawlessly.
So it's no surprise when Braddock gets his just desserts in the end. After first trying to bribe God (see "Symbols, Imagery, Allegory"), Braddock ends up dying in the explosion of his own giant diamond mountain. It's fitting that actually goes inside the mountain to die – it's is his prison so he cannot leave it, even in death. It's also fitting that he goes in voluntarily (just as he's voluntarily committed himself to a prison of his wealth), and that he leads his family in there with him – Braddock is a victim of his own volition.