Answer: The correct answer is : Because the Sheriff is trying to protect Boo Radley, because Boo tried to save the children and defend them and end up with a problem in the city.
Explanation: Boo Radley killed Bob Ewell with the same knife that Ewell was going to use to kill Jem or Scout. The Sheriff said that Ewell's Nut had been an accident and that he had fallen on his own knife.