Bob Ewell was obviously not satisfied with the guilty verdict against Tom Robinson and Tom's eventual death. Bob knew that Atticus had humiliated him and Mayella, and he wanted revenge. He was heard to comment after Tom's death that "it made one down and about two more to go"--probably meaning the two to go were Atticus and Judge Taylor. But Bob's cowardly streak precluded him from attacking Atticus, and he must have decided that killing Jem and Scout would be a satisfactory compromise. Bob must have been lurking about the school during the Halloween pageant and decided to follow the children home. He waited until they were in a dark area between the school and the Radley House on the edge of the deer pasture before attacking them. It would have been easy business for Bob, even though he was drunk: Scout was still wearing her cumbersome ham costume, and Jem was too small to handle Bob. But there was one more man lurking about that Bob hadn't counted on, and for Jem and Scout, it was the perfect time for Boo Radley to finally show his face.
Bob Ewell was an angry man. He was filled with rage, and he didn't care who he took it out on. He was extremely angry at Atticus. Atticus had not only defended Tom Robinson, but had also all but said that Bob was the guilty one. Bob was furious that Atticus would defend a black man in the first place, but he was also mad because he couldn't intimidate Atticus. Most people were afraid of Bob, but Atticus was not one of them. Bob was trying to find a way of getting back at Atticus, and he saw Jem and Scout as the perfect way to get to Atticus.
When Bob attacks Jem and Scout, he is showing us just what a coward he is. He is taking all of his anger for Atticus out on Atticus's children. This is not the first time that Bob mistreats children; he did the same thing to his daughter, Mayella. Bob thinks he has the upper hand with Jem and Scout, and that he is finally going to get his revenge on Atticus. The one thing Bob does not count on is there being another person there, a person who has been hidden in the shadows, but for the sake of Jem and Scout is now finally ready to come out into the light.
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.