It was Miss Stephanie’s pleasure to tell us: this morning 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.
– Harper Lee
To Kill a Mockingbird, Chapter 22. What Ewell said in his threat does indeed foreshadow that he will get back at Atticus – by attacking his children. Also that it will cost Ewell his life.