When Atticus tells his children that "it is a sin to kill a mockingbird," he does not give much explanation. Scout asks Miss Maudie, and she explains his reasoning by saying that all mockingbirds do is provide beautiful music. They do not harm anyone, they don't bother anyone, and they "sing their hearts out for us." As the previous poster stated, many characters in the book are "mockingbirds." Boo Radley is perhaps the strongest example because he wants nothing more than to keep to himself. When he saves Jem and Scout from Bob Ewell, he risks becoming the town hero, which is something he does not want. If Atticus and the rest of the people present at the time told the truth about what Boo did, they would be, essentially, killing a mockingbird. Boo did only good for them and they would turn his life into something he does not want.
Atticus tells his children that it's a sin to kill a mockingbird to develop a theme in this book. Tom Robinson is considered a mockingbird because all he was trying to do was help out Mayella Ewell who he felt sorry for. Tom Robinson was eventually sentenced guilty and was later killed. This was a sin. Boo Radley was also a mockingbird. He tried to interact with his commnity. Examples of this interaction is when he puts thinngs in the oak tree, also when he draped a blanket around Scout. He was really nice; he saved Scout and Jem from Bob Ewell. All throughout his life, he was locked inside his house. This was also a sin.
A mockingbird does nothing but sing and they are innocent. This also represents the innocence in the town that was shifted because of the stereotypical nature. Boo Radley is one because in fact he is not at fault that he is recluse, but because of the stereotypical nature of society, they have warped their views and changed him into something he is not.
i think it actually might be miss madie that explains why. basically she says that its a sin to kill a mockingbird because all they do is sing their hearts out. they dont harm anyone and they make people happy.
** mocking bird***