The reclusive Boo Radley kills Bob Ewell to protect Scout and Jem.
When Boo Radley kills Bob Ewell, he does so in order to defend his precious children. Boo Radley is a recluse who has befriended Scout and Jem over the course of the last few years because they are his neighbors, and because they are the only ones who actively tried to make friends with him. He started by just leaving them gifts, such as the watch and soap dolls, or looking out for them, such as putting the blanket on Scout’s shoulders at the fire and sewing Jem’s pants. However, his protection turned more pertinent when he noticed that a drunken Bob Ewell was following his children at the Halloween pageant.
Bob Ewell was angry at humiliated when Atticus proved that his daughter lied in accusing Tom Robinson of rape, hinted that Ewell himself was inappropriate with his daughter, and got Tom Robinson to say in open court that he felt sorry for Mayella. The worst, of course, was that the jury actually deliberated. Bob Ewell wanted revenge. He decided to go after Atticus’s children in a drunken rage.
Scout is wearing a ham costume, and cant’ see much. Jem is attacked first, and then Scout. She has no idea what is going on when Jem tells her to run, and she tries but does not get far.
He slowly squeezed the breath out of me. I could not move. Suddenly he was jerked backwards and flung on the ground, almost carrying me with him. I thought, Jem's up. (Ch. 28)
It was not Jem who rescued her though, it was Boo. This is not revealed until much later. When Scout is at home, she at first thinks Jem is dead. She does not know what really happened until she sees Boo sitting in the corner. She then tells her father what happened, putting the pieces together.
"Anyway, Jem hollered and I didn't hear him any more an' the next thing- Mr. Ewell was tryin' to squeeze me to death, I reckon... then somebody yanked Mr. Ewell down. (Ch. 29)
At first she points to Boo, saying, “Hey,” as casually as she can. Then she stops pointing in case Atticus will scold her. Atticus corrects her, telling her to call him “Arthur.” Dr. Reynolds does not seem to skip a beat upon seeing him, and this surprises Scout, who has trouble imagining the mystical figure ever getting sick.
Heck and Atticus have a discussion about whether or not they should tell anyone what happened. After all, Arthur Radley is a very private person. He would not want anyone to know.
To my way of thinkin', Mr. Finch, taking the one man who's done you and this town a great service an' draggin' him with his shy ways into the limelight- to me, that's a sin. (Ch. 30)
It is better for everyone to say that Bob Ewell fell on his knife trying to attack the children, and leave Arthur Radley out of it.
Scout gets to play out one of her fantasies, and walk Boo Radley home. She takes his hand as if he were a child. He is pale and very insecure and soft-spoken, but he asks her to take him home, after stopping to make sure Jem is all right. After walking Boo home, Scout notices something about Boo Radley.
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough. (Ch. 31)
Just by standing on the porch, she understands why he protected her against Bob Ewell. She sees her childhood through his eyes.