Bob Ewell hated Atticus Finch. He hated that he was a white man who defended a black man. He hated that Atticus said that both he and his daughter, Mayella, were lying about what happened to her. Atticus asked Mayella to admit in court that her father beat her up. She refused to do so.
Even though Tom Robinson was convicted of the crime, this was not enough for Bob Ewell. Shortly after the trial, Bob Ewell came across Atticus outside the post office. He spat in Atticus' face and "told him he’d get him if it took the rest of his life." Jem became worried about his threat, but Scout was less concerned.
Near the end of the novel, Jem and Scout walked home one night from a school pageant. Scout was wearing a bulky ham costume, which made it hard to move and even see. Jem thought he heard something. They continued walking. They sensed that they were being followed, but were not overly concerned. Suddenly Jem told Scout to run. Scout fell, trapped in her costume. Outside, she heard the sounds of a struggle. Bob Ewell had attacked Jem. Scout continued to hear the sounds of struggling. Jem escaped and tried to drag Scout away, but was attacked again. Boo Radley eventually came along and fought off Bob Ewell. He carried Jem away and Scout followed. Later, they found out that Bob Ewell was found on the "ground under that tree down yonder with a kitchen knife stuck up under his ribs." The man was dead.