Who or what killed the mockingbird in the book "To Kill a Mockingbird"

The sheriff arrives at the Finch house to announce that Bob Ewell has been found dead under the tree where the children were attacked, having fallen on his own knife. By this time, Scout realizes that the stranger is none other than Boo Radley, and that Boo is actually responsible for killing Ewell, thus saving her and Jem's lives. In spite of Atticus' insistence to the contrary, the sheriff refuses to press charges against Boo. Scout agrees with this decision and explains her understanding to her father. Boo sees Jem one more time and then asks Scout to take him home, but rather than escort him home as though he were a child, she has Boo escort her to his house as a gentleman would.

With Boo safely home, Scout returns to Jem's room where Atticus is waiting. He reads her to sleep and then waits by Jem's bedside for his son to wake up.