Bob Ewell despises Atticus for embarrassing him and presumably ruining his reputation during Tom Robinson's trial. During the trial, Atticus proved that Bob Ewell viciously assaulted his daughter after witnessing her kiss Tom Robinson. Following the trial,towards the end of chapter 22, Bob Ewell assaults Atticus by spitting in his face outside of the post office. In chapter 23, Scout recalls Miss Stephanie Crawford's story about Bob Ewell's confrontation with Atticus. According to Miss Stephanie, Bob Ewell spit in Atticus's face, challenged him to a fight, and proceeded to call him derogatory names. Despite Bob Ewell's aggression and hostility, Atticus refused to fight and simply wiped the spit off of his face before walking away.
After the confrontation, the children fear for their father's life, but he assures them that Bob got everything negative out of his system that day he confronted him outside of the post office. Unfortunately, Atticus dramatically underestimates Bob's wicked nature. Following the Maycomb Halloween festival, Bob Ewell seeks revenge on Atticus by attacking his innocent, defenseless children.