The characters of Light in August get swept up in an allegory that begins determining all of their fates. The allegory centers around the (supposedly) interracial relationship between Joe Christmas and Joanna Burden. Christmas and Joanna are both deeply traumatized people, and each of them has a very specific and strange relationship to both Blackness and Whiteness. Christmas may or may not have some Black ancestry, and Joanna may or may not have all White ancestry; it depends on the definition of "White" and the exact ancestry of her grandmother.
Once the rest of the world perceives them as a Black man and a White woman whose sexual relationship ends with Joanna's death, their story turns into a symbol for something that is not really about either of them. Suddenly, they are not only themselves. They are also a pair of stereotypes doomed to act in a certain way. Joanna is already dead, but Christmas must now go on the run from an angry White mob. He must figure out what to do with the murder weapon no one will believe Joanna intended to use on him. Stereotypes have such force in the community that the truth no longer matters. From the moment Brown announces that Christmas is Black, the townspeople begin to turn into an angry White mob that is ready to lynch Christmas. They almost seem helpless to stop themselves from turning the strange story of what has happened in their town into one they already know. Joe Christmas dies as a Black man who is targeted by White people. At the same time, he dies as a man who may not be Black at all.
Faulkner's use of allegory captures the effect of anti-Black propaganda that was spreading like wildfire in the South at this time, especially the false idea that Black men were a threat to White women. Films like Birth of a Nation normalized and even celebrated White mob violence against Black men as a kind of vengeful chivalry. The characters in the novel are so steeped in this false narrative that they can't stop themselves from playing it out for themselves.