The novel's climax occurs on Halloween night after the school pageant where Scout played a ham in the agricultural pageant (one of the novel's quietly humorous touches—Scout, dressed as a ham, misses her cue and ruins the show). Walking home in darkness, Scout and Jem hear someone following them through the trees.
Bob Ewell, still bitter about being humiliated during the trial, attacks the children with a kitchen knife. Jem fights to protect Scout, and his arm is broken badly in the struggle. Scout, trapped in her wire ham costume and unable to see, hears struggling, something hitting the tree, then silence. When she finally escapes the costume, she finds Jem unconscious and a man lying dead under the tree—Bob Ewell, killed with his own knife.
A mysterious man carries Jem home. Scout initially doesn't recognize him, but slowly realizes it's Boo Radley—the reclusive neighbor who hasn't been seen outside in years. Boo had been watching over the children protectively all along, and when Bob Ewell attacked them, Boo came out of his house and saved their lives, stabbing Ewell with Ewell's own knife.
Sheriff Heck Tate arrives and examines the scene. Atticus initially thinks Jem killed Bob Ewell in self-defense and wants Jem to face trial properly rather than have it covered up—even when defending his own son, Atticus insists on following legal process. But Sheriff Tate corrects him: Boo Radley killed Bob Ewell, and Tate intends to report that Ewell fell on his own knife.
Atticus objects, worried about lying to protect them, but Sheriff Tate explains that putting Boo Radley through a trial, even for justified homicide, would destroy the shy man who has hidden from public attention his entire adult life. "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... It's a sin and I'm not about to have it on my head."
Scout understands: "Well, it'd be sort of like shootin' a mockingbird, wouldn't it?" This connects to Atticus's earlier lesson that it's a sin to kill a mockingbird because mockingbirds don't do anything but make music—they don't harm anyone, just bring beauty. Tom Robinson and Boo Radley are both mockingbirds: innocent people who helped others and were destroyed (or nearly destroyed) by the community's cruelty. The sheriff's decision to protect Boo from public attention is the moral choice, even if it requires lying.
Scout walks Boo home, and he disappears behind his door, never to be seen again. Standing on the Radley porch, Scout looks out at the neighborhood and thinks back over the past three years, finally seeing events from Boo's perspective. She understands Atticus's central lesson: "You never really understand a person until you consider things from his point of view... until you climb into his skin and walk around in it." From Boo's porch, she sees herself and Jem playing, fighting, growing up—watched over by a kind man they called a monster.
The novel ends with Scout returning home to find Atticus sitting beside Jem's bed, reading aloud. Jem is asleep, his arm badly broken but healing. Scout crawls into her father's lap, and he carries her to bed. The final image is domestic and gentle: a father reading to his sleeping son, his daughter safe, the children protected by both their father's moral courage and a reclusive neighbor's hidden kindness.
The ending resolves the Boo Radley mystery (he was never a monster, just shy and kind) while leaving the larger injustice unresolved. Tom Robinson is still dead. Racism still exists. The court system still fails Black people. Bob Ewell's death doesn't fix the South's racism. But the novel ends on hope: Scout has learned empathy and justice. Jem, though disillusioned, has learned that some people (like Atticus) fight for right even when losing. And even in this racist town, there are people capable of kindness, courage, and moral choice. The mockingbird's death cannot be undone, but the sheriff's choice to protect Boo shows that individuals can still choose mercy within an unjust system.