The small Southern town of Maycomb has its own caste system, as Scout narrates in Chapter 13; promptly after her arrival Alexandra explicates the social strata of Maycomb, stereotyping all the old families of the area.
...they took for granted attitudes, character shadings, even gestures, as having been repeated in each generation and refined by time.
For instance, Aunt Alexandra attributes Sam Merriweather's suicide to a hereditary mean streak, and other families have their "streaks" that cause them to be certain ways. After Scout and Jem accompany Calpurnia to her church, Aunt Alexandra tells the children that they are not to return. Then, in Chapter 23, when Scout says she would like to invite Walter Cunningham to come to the house, her aunt quickly nixes this idea despite her concurrence that the Cunninghams are "good" people. And, of course, others in Maycomb display certain social biases, including Mrs. Dubose, who objects to Atticus's defense of Tom Robinson, the lowest of all on the social stratum of Maycomb.
Perhaps Bob Ewell, more than any other character, demonstrates the innate desire of people to want to feel superior to someone else. As low on the social scale as the Ewells are, there are no other white people that are beneath them. So, when Bob catches Mayella having invited Tom Robinson into the house, he feels that he must make certain that no one thinks that they have descended any further by crossing racial lines. Then, after the trial in which Bob and Mayella feel that Atticus has insulted them and tried to cause them shame in front of the townspeople, Bob vows to avenge himself against Atticus and Judge Taylor. One day before the courthouse, he spits in the face of Atticus and insults him, saying he is "Too proud to fight, you n*****r-loving bastard." But, Atticus does not give Ewell his anticipated reaction; instead he dryly replies, "No, too old."