When Atticus states that it is a sin to kill a mockingbird, instructing Jem and Scout never to shoot at them with their new air rifles, Scout wonders why this is so. She asks her neighbor Miss Maudie why killing this bird is a sin. Miss Maudie replies that this is because mockingbirds don't do any harm. They simply sing songs that people enjoy hearing.
The novel is called To Kill a Mockingbird because the mockingbird is a symbol of Tom Robinson. Robinson does no harm to anyone. In fact, he is helpful to Mayella Ewell, who he feels sorry for as a lonely young woman. He is also known in the Black community as an upstanding family man. Nevertheless, he is accused and put on trial for raping Mayella.
Atticus is able to prove the accusation is false and that Robinson is innocent. Nevertheless, Robinson is convicted of the crime because he is Black, and the Jim Crow code in the South says that the word of a white woman is always believed over the word of a Black man. Robinson is later killed. It was as much a sin to have killed this innocent man who did no harm as it is to kill a mockingbird.