How might Bob Ewell's words to Atticus foreshadow what is to come later in the novel?

Upon encountering Atticus at the post office, Bob Ewell spits in his face and tells him "he'd get him if it took the rest of his life." Since Bob is known to be an abusive drunk, and since Atticus gave evidence at Tom Robinson's trial that indicated Bob was the person who beat Mayella up following her interaction with Tom, Atticus is aware that Bob holds a grudge against him. The reader can assume this interaction is an indication of things to come since it highlights unfinished business and introduces a sense of foreboding into the relationship between Bob and Atticus. Atticus accepts the threat and ignores the insult, hoping that if he meekly refuses to fight back, Bob's rage will be temporarily quenched and Mayella will be spared a beating from her father. Since Atticus is levelheaded and attempts to treat everyone with dignity and compassion, he fails to see the impending danger in Bob's ongoing grudge against him.