When Jem and Scout are attacked on their way home from the festival, Jem has his arm severely broken. He is not able to defend himself or Scout. Scout tells Atticus and the sheriff,
"Somebody was staggerin' around and pantin' and--- coughing fit to die. I thought it was Jem at first, but it didn't sound like him, so I went lookin' for Jem on the ground. I thought Atticus had come to help us and had got wore out..." (pg 270)
She saw a person carrying Jem toward the house. She points at a man behind the door and identifies him as the person who helped the two children. It was Boo Radley.
The sheriff had already told Atticus that Bob Ewell was dead. He was under some trees with a kitchen knife stuck just under his ribs. For some reason, Atticus thinks Jem killed Bob Ewell. He somehow got the knife out of Bob's hands and used it against him. He talks to the sheriff about pleading self-defense in court for Jem. Mr. Tate clarifies the situation
"Jem never stabbed Bob Ewell." (pg 272)
Sheriff Tate then goes on to tell Atticus that he is going to write this up as Bob Ewell falling on his knife. Atticus objects stating that he doesn't want his son to grow up with this crime hanging over his head. He wants him to face it straight on. Sheriff Tate then explains to Atticus that it wasn't Jem who killed Bob Ewell. He never says the person's name, but it is strongly inferred that Boo Radley did the deed. The sheriff is going to write it up as Bob Ewell falling on his knife because,
"Mr. Finch, taking the one man who's done you and this town a great service and draggin' him with his shy ways into the limelight ---to me that's a sin.....If it was any other man it'd be different. But not this man, Mr. Finch." (pg 276)
Sheriff Tate is talking about Boo Radley. Boo came to the aid of the children and made sure that Jem was brought home for medical care. He was the man carrying Jem home.
After the sheriff leaves, Atticus tells Scout that what they are going to say is that Bob Ewell fell on his knife so that Boo would not be bothered with people wanting to thank him and honor him for saving the children. He asks her if she understands. She assures him that she does.
"Well, it'd be sort of like shootin' a mockingbird, wouldn't it?" (pg 276)
Atticus had earlier told her she should never shoot a mockingbird because,
"Mockingbirds don't do one thing but make music for us to enjoy....they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird." (pg 90)
Boo did nothing wrong. He protected the children and his life did not need to be turned upside down for being so kind.
At the of Chapter 30, Atticus said to Boo Radley,
"Thank you for my children, Arthur." (pg 276)
Arthur is Boo's real name.