In Chapter 10, Scout recalls the time Atticus gave her and Jem air-rifles but would not teach them to shoot. This is interesting because Scout and Jem will later learn that Atticus was a superb marksman. Uncle Jack taught them instead. However, Atticus did instruct the children that if they did go shooting at birds that they could shoot blue-jays but not mockingbirds. He added that it is a sin to kill a mockingbird.
Scout then asked Miss Maudie about it. Miss Maudie replied:
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.
This becomes a theme for the entire novel. It is a sin to kill or harm something or someone who does nothing but offer beauty, help, and innocence to the world. Therefore, it would be a sin to harm Boo Radley and Tom Robinson, (and Atticus, Jem, and Scout for that matter) because these are people/characters who do nothing wrong and in some cases, they offer to help others.