In Chapter 10, Atticus makes the following statement: “Remember it’s a sin to kill a mockingbird.” Scout asks Miss Maudie her opinion on this and the woman responds, "Your father’s right . . . Mockingbirds don’t do one thing but make music for us to enjoy . . . but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.”
The larger principle suggests a theme of good versus evil--of good things, like mockingbirds who commit no wrong, being destroyed by bad things. For instance, this can be applied to Boo Radley. He is a "good" character--he helps the children, leaves them gifts, and so forth, and yet because of his dark history they are frightened of him. Additionally, his abusive past indicates the "evil" aspect of the theme.
The book develops this further in later chapters, but basically it comes down to the issue of protecting the weak--the characters (Atticus and Scout in particular) prove by word and deed that it is necessary to protect those who cannot protect themselves.
Earlier in the novel, the children are playing with their air rifles and Atticus tells them that it is a sin to kill a mockingbird. Miss Maudie elaborates by telling Jem and Scout that mockingbirds are sweet birds that do nothing to harm anyone and continually sing throughout the day. Throughout the novel, mockingbirds symbolize innocent beings like Tom Robinson and Boo Radley. The larger principle involved concerns the importance of defending innocent beings who are unable to defend themselves. Atticus does his best to defend Tom Robinson against the prejudiced community of Maycomb. Also, Sheriff Tate refuses to inform the town about Boo Radley's heroics because bringing attention towards Boo would harm him. Both Tom and Boo are symbolic mockingbirds because they are innocent beings who cannot defend themselves throughout the novel. Jem and Scout learn the importance of protecting vulnerable and innocent people after witnessing their father defend Tom Robinson.
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