Why does Bob Ewell feel so angry with Atticus? Is this threat real and how might he try to "get" Atticus?

Atticus has hurt Bob's pride. His threats against the Finch family are real, as evidenced in later chapters of the text.

Bob Ewell is angry with Atticus becuase he publicly humiliated him on the witness stand during the trial.

Yes his threats against the Finch family are real, will be proven in the other chapters