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

Which specific words are you referring to? Are you referring to the following quote?

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.

If so, this quote deifintely foreshadows Bob Ewell's actions in the rest of the novel. He means to get back at Atticus in anyway he can. This quote foreshadows the attack on Scout and Jem.