Who really killed Bob Ewell? How do we know? What happens to the killer and why?

In chapter 30, Atticus Finch, Mr Tate, Scout and Boo Radley people discussed who killed Mr Ewell, including suspicion of Jem Finch. Later on in the chapter they figure out that jem did not murder Mr Ewell, but it is said Boo Radely was at the scene and was the man who is said to be the murder of Bob Ewell, but as they talked, they had changed the story and it had been said that Bob Ewell had fallen into a knife he was going to use to kill Scout and Jem, when Boo Radely arrived at the scene he saved Jem and Scout. The sheriff Mr Tate thought it would be unnecessary to tell the town as Boo Radley was a shy man and "Well, it'd be sort of like shooting a mocking bird wouldn't it"

-Brooke, Chenae and Eliza.