Scout has moved on and is killing people now. She will be the one on trial, and her father will be the prosecuting attorney. His honor requires that he seek the death penalty against his daughter.
It also turns out that Boo Radley's killing of Bob Ewell wasn't in defense of Jem and Scout, but was actually just the first in a long line of brutal murders. By the events of this novel he is known as the Maycomb Mauler and is believed to have killed as many as 900 people. Atticus tries several times to shoot him with his trusty rifle, but he takes on the legacy of the one thing "One Shot Finch" couldn't hit. Everyone lives in terror that he will come to their house at night.