Why is it a sin to kill a mockingbird?

Atticus tells Scout and Jem they can shoot their air guns at tins cans and bluebirds, but that it is a sin to kill a mockingbird. Miss Maudie affirms this, saying to Scout, "Your father's right. Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird."

Atticus's warning about shooting a mockingbird is the first reference to the novel's title and mockingbird theme. Atticus doesn't want his children to inflict cruelty upon the innocent mockingbirds just because they have the power to, just as he doesn't like to shoot for sport. His warning serves to emphasize the responsibilities that come with power. Those who have power must be careful not to use it cruelly against the innocent and harmless. The powerful must be careful in choosing whom they target.