in Chapter 27:Describe the actions of Bob Ewell. What was he up to and what was motivating him?

Last sentence in the chapter:  "Mr. Bob Ewell stopped Atticus on the post office corner, spat in his face, and told him he'd get him if it took the rest of his life."  Spitting in the face is an insult; it was meant to show Ewell's contempt for him. From Ewell's point of view, Atticus insulted him but implying Ewell, not Tom, assaulted Mayella.  The subsequent threat proves true at the end of the novel when Ewell attacks Scout and Jem while walking home in the dark.