Why do you think it was important to Mr. Ewell to get revenge on Atticus? What does this tell us about him as a person?
Bob Ewell seeks revenge on Atticus because, in Ewell's opinion, Atticus humiliated him on the witness stand. In exposing the truth behind the Tom Robinson case, Atticus showed Tom did not beat or try to rape Mayella; instead, Atticus suggested that Ewell, Mayella's own father, beat and abused his daughter. This revelation came in a very public setting, so Ewell believes Atticus ruined his reputation in the eyes of Maycomb. Since Ewell is so concerned with his own reputation, we can assume he is an accordingly shallow individual who cares only for surface appearances.
Ironically, Ewell already had a lousy reputation. Known as a shiftless, irresponsible drunkard who did little to care for his large and impoverished family, Ewell didn't have a good reputation before the trial. As such, it's ironic that he should be so concerned with seeking revenge on Atticus for damaging his reputation, as he never had much social esteem in the first place.