One of the hardest jobs for an author is to choose a good title for their writing. It has to be catchy, relevant to the story, and hints at the content of the story without revealing too much. Harper Lee does this successfully by titling her novel "To Kill a Mockingbird". Besides evoking interest in the potential reader, the title is very significant and symbolic to the story. Atticus tells his children that it is a sin to kill a mockingbird and Miss Maudie explains "'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.'" (P. 90) The mockingbird represents characters and events: Tom Robinson, Boo Radley, the justice system, and childhood innocence.
Tom Robinson is a gentle man who, like the mockingbird, has never harmed anyone and only wanted to help Mayella Ewell because he felt sorry for her circumstances.
His incarceration and subsequent death was not unlike killing a harmless animal. "Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of songbirds by hunters and children." (P. 241)
I think that Harper Lee deliberately did not restrict the mockingbird metaphor to the black race. The mockingbird also represents Boo Radley who was a member of the white society. He is a shy, fragile man who is also in danger of "being killed" if he were to be put on trial. His only crime was protecting innocent children from the attack of Bob Ewell. When Atticus tells Scout that the story will be that Bob Ewell fell on his knife, Scout replies, "'Mr. Tate was right...Well, it'd...