Within the Mockingbird motif of Harper Lee's novel falls the roly-poly, and the mockingbird, who are innocent creatures that bother no one. However, Tim Johnson, the rabid-dog, is a clear threat to the health and well-being of the neighborhood; therefore, Atticus has no choice but to destroy it.
The symbol of the mockingbird represents innocence. Such characters as Boo Radley, Mr. Dolphus Raymond, and Tom Robinson are mockingbirds because they intend no harm to anyone in the town; clearly, there is no justification in harming any of these three men. Tom's death represents the destruction of innocence by the evil of Maycomb's racial prejudice. Likewise, the songbird and the roly-poly are harmless, and to kill them is a cruel and mean act. Perhaps to Jem who is reeling from the amazement at the cruelty of the jurors, this roly-poly symbolizes poor Tom Robinson, who meant no harm but was unthinkingly destroyed. And, Scout's hand that is ready to end its life symbolizes the Ewells who callously destroy Tom.
With the episode of Tim Johnson, Miss Lee demonstrates that there must be a justification for people's acts. For, gratuitous cruelty is always wrong.