Heck Tate Convincing Atticus that Jem is Innocent
Bob Ewell fell on his knife. Jem didn't kill him.
Keeping Boo Radley out of the Spotlight
I know Arthur (Boo) stabbed Bob Ewell, but we can't tell people.
Atticus thinks that Jem stabbed Bob Ewell and is ready for Jem to take responsibility for his actions.
No Angel Food Cake for Boo Radley
Boo doesn't like attention. If everyone knew, they would bring him gifts and he would be uncomfortable.
Sheriff Heck Tate is trying to tell Atticus that Bob Ewell accidentally killed himself by falling on his own knife. He knows Jem didn't kill Bob Ewell and is trying to convince Atticus that Jem is innocent.
Atticus believes Heck Tate when he says that Jem didn't kill Bob. They know that Boo Radley actually killed Bob Ewell to protect Jem and Scout, but they can't tell anyone because they know that Boo doesn't like attention.
They don't want Boo to be uncomfortable with all the attention he would recieve.