How can Thomas Sutpen's story can be interpreted as an allegory of the South itself.

"Absalom, Absalom!" is a detailed story about Thomas Sutpen, who moves to the south to gain wealth and power. He creates a plantation, gets married, and takes on slaves—eventually amassing all the wealth and luxury he intended to gain. His plan having succeeded, things start to go awry when a child from a previous marriage, when he was in the West Indies, shows up unexpectedly and tries to woo his daughter.
A myriad of terrible events befall him and his family because of his former actions, much of this happening during and immediately after the Civil War.
Allegorically speaking, this story portrays the bigger picture of the South at large. White landowners took advantage of the South, amassing large estates and essentially creating dynasties and enforcing their will on everyone around them, only to have it stripped from them with the righteous liberation of the slaves during the Civil War. His former relationship in the West Indies is a portrayal of the old slave trade, and how its descendants were the undoing of slave owners in the South. When they were liberated, the economy of many of these plantations faltered, and they were able to rise up and overthrow their former oppressors.
The inbred, violent society of the South reaped the reward for what it had sown. Its greedy and perverse nature caused it to amass too much and destroy itself in the process by trying to control everything and oppress others around it.