What do you think is the real reason why Bob Ewell is so intent on getting even with Atticus? Why is Mr.Ewell so threatened by Atticus after the trial?

Bob was humiliated by Atticus. Although the verdict went his way, the truth about what happened was plain to see. Bob felt Atticus made him look like a stupid hillbilly which of course was not hard to do. The very disturbing fact about who he fathered all those children with was also evident to the town. Bob wanted to save face the only way he knew how.