Why does Atticus insist to Scout that Ewell fell on his knife? What does she compare it to?

Atticus thought that Boo stabbed Ewell which was probably true. Heck Tate however convinced Atticus of how nobody would win if the truth came out. Boo, a recluse, would be destroyed by the publicity and attention. Atticus finally agrees it better to say the drunk Bob Ewell fell on his own knife. Scout understands that making Boo Radley a hero would destroy him: "Mr. Tate was right....Well, it'd be sort of like shootin' a mockingbird, wouldn't it?"