To kill a mockingbird, according to Scout's father, Atticus, is a sin because the mockingbird does no harm. All it does is sing. This symbol is connected to two characters in the book, Boo Radley and Tom Robinson, both mockingbirds in their own way. Boo Radley scares the townspeople, yet he has done none of the things that the gossips in the town say he has. His biggest crime was to have the father he has. He has become strange through the experience, but he is also an innocent. To harm Boo would be similar to harming a mockingbird. Tom Robinson is an innocent victim of the town's biggotry. In this, he, too, is like a mockingbird. Both Boo and Tom do acts of kindness (like birdsong) that result in their potential to be harmed. Boo's act is at the end of the novel when he saves Scout and Jem. Tom's act is to help Mayella. Boo does not end up being charged because both Atticus and the Sherif agree that it would harm him. Tom, however, is actually destroyed. Which is more than a shame--it's a sin.
The mockingbird symbolizes innocence and generosity. In Chapter 10, Atticus tells the children that it is a sin to kill a mockingbird. Scout notes that this is the only time Atticus said it was a sin to do something. Miss Maudie clarified his point, saying, "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. That’s why it’s a sin to kill a mockingbird.” Eventually, the mockingbird comes to symbolize two characters in particular: Tom Robinson and Boo Radley. Tom helps people (Mayella) when they need it but otherwise keeps to himself. Boo keeps to himself more than most but helps the children without them even knowing it. So both Tom and Boo are like mockingbirds in the sense that they never harm anyone and, in fact, they help people, neither expecting recognition or a reward.