After Tom Robinson's trial and conviction, Bob Ewell was enraged at Atticus because both he and Mayella had been so vigorously cross examined by Atticus in his defense of Tom. When Ewell and his daughter were on the witness stand, Atticus through his questioning showed them both to be liars. Ewell was furious because he had been humiliated by the truth, and he directed all his fury at Atticus.
When Ewell found Atticus on the corner by the post office, Ewell spat in his face, tried to start a fight, and threatened to kill him. According to witnesses, he told Atticus he would "get him," regardless of how long it took. Atticus, however, ignored Ewell's taunts and refused to fight with him.
However, Bob Ewell did not follow through on his threats by attacking Atticus. Instead, being the coward he was, he tried to get even with Atticus by attempting to kill his children.