The racism of the white ladies of Maycomb whom Aunt Alexandra likes to entertain is based, at least in part, on self-interest. The local African Americans are a good source of cheap labor for these ladies, and the women don't want them to start getting "uppity." Therefore, they are eager to support a system that uses racism to hold them down. Mrs. Grace Merriweather, for example, attacks Atticus for his defense of Tom Robinson by saying,
... all they did was stir ’em up. That’s all they did. Might’ve looked like the right thing to do at the time, I’m sure I don’t know, I’m not read in that field, but sulky… dissatisfied… I tell you if my Sophy’d kept it up another day I’d have let her go. It’s never entered that wool of hers that the only reason I keep her is because this depression’s on and she needs her dollar and a quarter every week she can get it.
In other words, Mrs. Merriweather is badly exploiting her black servant Sophy—even in the Depression, a wage of $1.25 a week was appallingly low (the sharecroppers the Joads, for example, in The Grapes of Wrath, are horrified when their pay goes to 10¢ an hour, which for a forty-hour work week would amount to $4.00), because the poor woman has no other option than to work for next to nothing. Mrs. Merriwether very much does not want this system, which benefits her fully, to change.
Another form of racism emerges with the Ewells, who, as the town's white trash, are able to keep their heads up by feeling superior to African Americans. They live next to the African Americans, in a lower-class part of town, the father is a shiftless and cruel alcoholic, the children are uncared for, filthy, underfed, and don't attend school, but Bob Ewell can at least feel that he is better than a "Negro." He, for personal reasons of pride, is bent on upholding the system of white supremacy. In fact, he is so angry that Atticus humiliates him by showing he was lying in court that he tries to kill Jem and Scout in revenge.
Racism also arises simply from a lazy and unquestioning acceptance of the worst stereotypes about African Americans. This is summarized by Scout in the common response to Tom Robinson's death:
To Maycomb, Tom’s death was typical. Typical of a nigger to cut and run. Typical of a nigger’s mentality to have no plan, no thought for the future, just run blind first chance he saw. Funny thing, Atticus Finch might’ve got him off scot free, but wait—? Hell no. You know how they are. Easy come, easy go. Just shows you, that Robinson boy was legally married, they say he kept himself clean, went to church and all that, but when it comes down to the line the veneer’s mighty thin. Nigger always comes out in ‘em.