What happens to Bob at the end of To Kill a Mockingbird?
You don't mention the name of the novel in your post, so I will assume you are speaking of Bob Ewell in To Kill a Mockingbird.
Although Tom Robinson was convicted of raping Bob's daughter, Mayella, and later paid for it with his life, Bob was not satisfied. He and his daughter had been humiliated under cross-examination by Atticus, and Bob swore revenge. Spitting in Atticus's face was not enough for Bob, who threatened to get even with Atticus "if it took the rest of his life." Bob was also angry with Atticus for "getting his job" when he was "fired from the WPA for laziness." Atticus was not his only target: Bob also stalked Tom's widow, "crooning foul words" at her as she walked past his house; and later, he was nearly caught prowling around Judge Taylor's back porch.
On Halloween night, Jem and Scout were suddenly attacked by someone on their way home from the school. Scout could not identify the attacker, since she was still wearing her cumbersome ham costume, but
Something crushed the chicken wire around me. Metal ripped on metal and I fell to the ground.
The "metal on metal" was Bob's knife trying to rip through the chicken wire, but soon Jem came to Scout's rescue, and he began wrestling with Bob. But Bob was soon on Scout again and
He slowly squeezed the breath out of me. Suddenly he was jerked backward and flung on the ground...
The man who had thrown Bob to the ground was none other than Boo Radley, who must have stabbed Bob to death after a struggle. Bob's body was found with "a kitchen knife stuck up under his ribs." Although Atticus at first thought that Jem was the killer, Sheriff Tate alluded that it must have been Boo, although the official version was that "Bob Ewell fell on his knife."