In Chapter 30 of To Kill a Mockingbird, Mr. Heck Tate insists that he report that Bob Ewell fell on his own knife and died, but Atticus Finch believes that Mr. Tate is trying to merely protect Jem. He tells the sherriff that he must have Jem live honestly and face the consequences of his action of stabbing Bob Ewell because he as a father has taught his children to do so. However, Mr. Tate insists that Jem could not have exerted the force with which the knife was thrust into Ewell's "craw":
"Your boy never stabbed Bob Ewell...didn't come near a mile of it and now you know it. All he wanted to do was get him and his sister safely home."
Then Mr. Tate tells Atticus that it is not against the law for a citizen to prevent a crime from being committed. If this citizen becomes known, all the townspeople would knock on his door and drag him "with his shy ways into the limelight....to me, that's a sin."
As Atticus sits on the porch, he finally raises his head to Scout who has recognized the phrase "that's a sin" from advice that Atticus has given her. When Atticus concludes, "Mr. Ewell fell on his knife," he asks Scout if she understands. Indeed, she does--thoroughly. Scout tells her father,
"Mr. Tate was right....Well, it'd be sort of like shootin' a mockingbird, wouldn't it?"
Reminded of his lesson about the use of bb guns, Atticus is assured that Scout truly does understand. He is convinced.