Corruption and Decline of the South
An interesting example of this comes from Shreve's description of the results of the Confederacy's methods of promoting generals based on social standing, not on ability, with generals who like to dress up but don't know anything about leading an army or tactics; as Shreve puts it, generals who capture warships but not food or ammunition, who defeat three separate armies but then tear down their own fences to cook meat robbed from their own smokehouses, who set fire to enemy garrisons but are then caught in bed with a neighbor's wife and are shot and killed, and so on. This shows how the leaders of the South don't understand their objectives, and don't understand the value of their own people or the people around them.
The story of Thomas Sutpen illustrates this decline. He goes to Haiti because he thinks that it is the quickest way to make a fortune. He manages to make his fortune, but he marries a woman who has African ancestry, so he feels he must bribe her with that fortune so that he can abandon her and his son. That Sutpen thinks that the problem will be resolved by merely leaving enough money behind suggests a strange perversion of values, especially considering how much Sutpen struggles to have a son later in his life. More than that, though, Sutpen's decline happens primarily because of this decision; his abandonment of his first son creates his greatest enemy (Charles Bon). A related decision to take advantage of Milly Jones makes Wash Jones, one of Sutpen's most useful allies after the war, into his killer. Sutpen's decisions to put his "design" ahead of concerns for family (Milly seems less important than his mare) or friends (ignoring Wash's concern for his granddaughter) destroys him. It is important, too, that much of Sutpen's wealth existed in human beings: when his slaves abandon him during the war, he loses a considerable part of his fortune, as well as the labor he needs to make his plantation a profitable business.
The Compson family also illustrates this decline, though less directly because they are less important to the story. General Compson is an important military leader in the Confederacy and an active member of the community, and his son a prominent citizen who doesn't seem to do very much other than run his plantation and read. Quentin attends Harvard, but he becomes obsessed with his sister and with the history of the Sutpen family, and he will commit suicide a few months after the setting of this novel (as described in The Sound and the Fury).
The novel makes it clear that these were not the innocent victims of a violent Northern aggressor; the South had become deeply invested in a system that encouraged the mistreatment of its own people and turned much of its population into objects for sale. More than that, though, people like Sutpen who put wealth and success ahead of everything else can become powerful in this system, with disastrous results.
Dangers of social mobility
There are several moments when Mr. Compson points out Sutpen's "breeding," and it seems important to Mr. Compson to mention, for example, that Sutpen's greeting of the other men in the town when he is crossing the square before he is arrested shows his lack of "breeding," and the way that Sutpen had instead practiced and memorized appropriate behavior (instead of it being natural or second nature, Mr. Compson seems to suggest). Mr. Compson also makes several remarks about Sutpen's overly formal language, pointing out that Sutpen had learned to speak like an aristocrat in the same way that he had learned his manners, through artificial study and memorization. Most importantly, Sutpen's "design" tries to reduce social and material success to an equation, with certain required elements, without regard to how those elements are obtained. Sutpen is a Machiavellian "ends-justify-the-means" type of person, and he doesn't seem to see the complete picture of potential consequences of his actions. Mr. Compson calls this Sutpen's "innocence," but it might be more fair to call it ignorance. Just as a language learner cannot be expected to ever master a language at the same level as a native speaker, so a social climber cannot be expected to master the rigorous expectations of the highly regulated and controlled upper social class. (Mr. Compson, as a kind of native speaker of the upper class, easily spots Sutpen's mistakes and awkwardness.)
It is interesting, too, how the "lower-class" characters express their outrage at Sutpen's rise, as if they don't like his success or don't feel that he deserves it. It's not the wealthy upper-class citizens of Yoknapatawpha who stand outside the church and pelt Sutpen and Ellen with dirt and vegetables, it's the working class and lower class of the county. And it's not someone like General Compson who kills Sutpen, it's Wash Jones, perhaps his greatest admirer. In other words, perhaps these are the people who are most invested in believing that the upper class deserves what they have, and the most outraged when the upper class behaves in a way that shows that they don't deserve it.
So, in other words, there are two kinds of "dangers" of class mobility. First, that the upper class will never completely assimilate someone who isn't born to that class and that both classes will try to expel unqualified newcomers, and second, the wealth and power that come from class position can become dangerous in the wrong (or "underbred") hands.
Absalom, Absalom!: Theme Analysis