Bob Ewell is angry because Atticus Finch has proven him a liar before the citizens of Maycomb who are present at the trial. His threat to "get even" with Atticus Finch is a real one and, because he is unconscionable, he will probably do something underhanded in order to avenge himself.
Oddly enough, the disreputable Bob Ewell, who is considered "white trash" by the Maycomb community because he is shiftless and unconcerned about the welfare of his motherless children, feels insulted by Atticus Finch when cross-examined by him. He also feels that Atticus has turned the jury and the others in the courtroom against him by drawing their attention to his ignorance and falsehoods, such as his claim that Mayella was beaten by the one-armed Tom Robinson, with his useless left arm, when the beating was clearly issued by a left-handed person.
After the trial, Bob Ewell is vociferous about his intentions to avenge himself against Atticus. One day on the town square, Ewell accosts Atticus, cursing him. Then he spits tobacco juice on Atticus's face and threatens to kill him. When Atticus does not retaliate, Ewell accuses him, "Too proud to fight, you n****r-loving' bastard?" Atticus drily replies, "No, too old." This passive response, foments the bellicose Ewell even further as the narrative later evinces.