All the King's Men is set mostly in the Deep South in the 1920s and 1930, and you don't need us to tell you that racism was rampant and harsh in that setting. Since all of the main characters in the novel are white, racism seems to be a minor theme in the novel. But by the time all the stories are told, it's clear that issues of racism and slavery are hugely important to the novel. The novel suggests that exposing historical racism, and exposing hidden secrets of antebellum (pre-Civil War) life are our only hope for overcoming racism.
All the King's Men offers us a fictional history of racism that helps us understand racism in real life.
When we realize that the Mason County boys are just using racism to cover up their greed, we understand that the extent to which greed can corrupt the human race.