One very strong hint of Bob Ewell's vindictive act is in his insulting actions and words in front of the post office: "Mr. Ewell approached him, cursed him, spat on him, and threatened to kill him "if it took the rest of his life." The reports of this incident occur at the end of Chapter 22 and the beginning of Chapter 23 of To Kill a Mockingbird.
Also,in Chapter 17 Scout narrates that Robert E. Ewell, "a little bantam cock of a man"--which indicates his attitude--takes the stand and regards Atticus with "haughty suspicion." And, while Bob Ewell in on the stand during the trial of Tom Robinson, he is asked to write upon an envelope. Unknowingly, he complies, but when he looks up, he discovers the judge staring at him as though he were "some fragrant gardenia in full bloom on the witness stand," and the jury watching him. Sensing that Atticus Finch has somehow acquired an advantage over him. Ewell becomes angry and declares that Atticus Finch is taking advantage of him. This anger of Ewell's and embarrassment in front of the courtroom crowd is not to be forgotten by Ewell. He tells the judge that "tricking lawyers like Atticus Finch took advantage of him all the time with their tricking ways." Even Atticus later mentions that Bob Ewell will not forgive him for exposing his ignorance.
On the night of the attack upon the children by Bob Ewell, there are foreshadowing of the danger. For instance, Scout notes that
a solitary mocker poured out his repertoire in blissful unawareness of whose tree he sat in...[the whippoorwill sang] Poor will, Poor Will.
Like the mockingbird, Scout is unaware of what will soon happen; she trips on a root growing in the road as her costume impedes her movements. then, when she and Jem turn off the road and enter the schoolyard is "pitch black." Scout tells Jem, "You should have brought the flashlight." And, Cecil jumps out at them, and asks them if they are not "scared of Boo Radley?" During the performance, little Scout falls ill. As she and Jem leave the auditorium, "it was still black dark," but they walk toward home even though Scout has trouble in her cumbersome costume. When Scout feels pressure on her neck, she thinks it is Cecil teasing again as they hear a sound like leaves blowing in the wind "only there wasn't any wind, and there weren't any trees beside the big oak behind them. At this point,they are attacked.
Later, in Chapter 29, Scout remarks,
Somehow, I could think of nothing but Mr. Bob Ewell saying he'd get Atticus if it took him the rest of his life. Mr. Ewell almost got him, and it was the last thing he did.
Those remarks of Bob Ewell's coupled with the dark, sinister night and the haunting sounds of the mockingbird and the whippoorwill, certainly suggest that ill is going to befall the Finches.