Atticus thinks that Jem stabbed Bob Ewell at first.
When Scout and Jem head back from the Halloween play, they are accosted by Bob Ewell. He attacks them with a knife. Scout is dressed as a ham, so she is uninjured. Jem's arm is badly broken. At the end of the scuffle, Bob Ewell is dead.
When Atticus hears what happened, he assumes that Jem killed Bob Ewell defending Scout.
Atticus pushed up his glasses and pressed his fingers to his eyes. "Jem's not quite thirteen... no, he's already thirteen- I can't remember. Anyway, it'll come before county court-" (ch 30)
Atticus describes it as a clear-cut case of self-defense. He still thinks that Jem is going to be arrested though. Heck Tate thinks differently. He is convinced that Boo Radley killed Bob Ewell to defend the children.
Boo Radley is a sensitive and shy man, and would not like anyone to know what he did. Heck Tate is aware of this, and decides to say that Bob Ewell fell on his knife.
This incident brings the book full circle, and ties together the two main plot lines. It also ties up two of the loose ends. It dispatches with Bob Ewell, and the threat he made to Atticus. It also clears up Boo Radley’s situation and verifies that he was watching the children and he is a good person.
Atticus is convinced that it was Jem who killed Bob Ewell during Bob's retaliatory attack upon the children.
In Chapter 30 of To Kill a Mockingbird, Sheriff Tate, Atticus, Scout, and Boo Radley sit on the porch while, in the bedroom, Dr. Reynolds sets Jem's broken arm. Atticus tells the sheriff that the incident of the night will be presented in county court.
"What will, Mr. Finch?"
"Of course it was clear-cut self defense, but I'll have to go to the office and hunt up—"
"Mr. Finch, do you think Jem killed Bob Ewell? Do you think that?"
Atticus does believe that Jem killed Bob Ewell. He tells Sheriff Tate that Scout said that Jem got up and yanked Ewell off her, and "he [Jem] probably took Ewell's knife somehow in the dark. . . ." When the sheriff cuts Atticus off and says, "Jem never stabbed Bob Ewell," Atticus thanks him but adds,
"Heck . . . I know you're doing it from that good heart of yours, but don't start anything like that. . . . nobody's hushing this up. I don't live like that.
"Nobody's gonna hush anything up, Mr. Finch."
Sheriff Tate argues the case for Bob Ewell's having killed himself for some time. Finally, he says, "Mr. Finch, Bob Ewell fell on his knife. . . . He killed himself. I can prove it." However, Atticus is still unconvinced, so Sheriff Tate pulls out a knife and demonstrates how Ewell could have fallen on his own knife. He finishes by saying, "Scout is eight years old. . . . She was too scared to know exactly what went on. . . . Your boy never stabbed Bob Ewell . . . didn't come near a mile of it and now you know it."
Sheriff Tate then stands before Atticus and clarifies his intentions. He tells Atticus,
"There's a black boy dead for no reason, and the man responsible for it's dead. Let the dead bury the dead."
In other words, Ewell cannot deny that he did not kill himself. By saying this, Tate implies that there is no reason to drag Arthur Radley into the murder. He explains that taking Arthur and his shy ways into the "limelight" would be wrong. "If it was any other man, it'd be different. But not this man, Mr. Finch." He refuses to let Arthur be charged with killing Ewell because Arthur was only trying to save the children.
After the sheriff departs, Atticus asks Scout if she understands Mr. Tate's reasoning in protecting Arthur. Scout says she does, adding that to make Boo testify would be like shooting a mockingbird. "Mr. Tate is right," she tells her father. Atticus turns toward Arthur and says, "Thank you for my children, Arthur."