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The main theme is "It is a sin to kill a mockingbird" in other words, picking on people who are helpless and don't do harm is a terrible thing. Some of the mocking birds (which is a symbol in the book) in the novel are, Boo Radely and Tom Robinson. The theme also represents the racial injustice in the South.
Predjudice and racism. Also innocence, especially for the Jem and Scout
Race in America, primarily racial inequality and gender roles in the south, are the focal points of To Kill a Mockingbird. When Atticus, a central character, is appointed to defend a black man who purportedly raped a white woman, the central themes of the book emerge: ethics and justice as they pertain to the Christian ideals set forth during the Great Depression in Harper Lee's fictive town of Maycomb Alabama.