Consider the title - To Kill a Mockingbird

Mockingbirds are creatures placed upon this earth with the soul purpose of creating beautiful music. They do no harm and are a nuisance to no-one, for 'they don't do one thing but sing their hearts out for us,'  and it is for this reason that it is a sin to kill a mockingbird. In the novel, To Kill a Mockingbird, the mockingbird symbolizes innocence and the sin found in accusing 'mockingbird' characters of crimes they did not commit. Thus, to kill a mocking pined is to destroy innocence
In this case, the two characters of Boo Radley and Tom Robinson could be identified as mockingbirds for, in their own ways, they are innocent but have been harmed through their contact with injustice. In Tom Robinson's case, it is very clear that he is indeed innocent of the crime he was accused, and yet he had to face the consequences of a crime he did not commit. When Tom Robinson is shot, Mr Underwood compares his death to 'the senseless slaughter of songbirds.' Boo Radley is also innocent, but in a completely different manner to Tom, and yet he is ostracized from society for reasons that are not entirely true. Even young Scout realises that hurting Boo Radlye would be like 'shootin' a mockingbird.' Both men have kind hearts and only wish help others, but are ultimately destroyed through the injustice of society - just as innocence is destroyed when a mockingbird is shot.

While the title of To Kill a Mockingbird has very little literal connection to the plot, it is deeply significant, and presents, to the reader, a number of major themes that underpin the novel. The importance of moral education is explored to a great extent throughout the novel, and the symbol of the mockingbird is key in portraying this idea. The story follows the growth of Scout and Jem in particular, and much of the exploration of moral questions take place from the children's perspective. However, this theme is most powerfully examined through the relationship between Atticus and his children. Atticus is a very morally upright man and thus, devotes himself to instilling a social conscience in both Jem and Scout. Atticus uses the symbol of the mockingbird to teach the children about innocence and the sin found in destroying it. It is through Atticus's lessons, where the children place themselves 'in someone else's shoes,' that both Jem and Scout come to understand the prejudices of society and realize that some people, like Tom Robinson and Boo Radley, are just like mockingbirds, and to hurt these people would be similar to hurting a mockingbird. However, through these lessons, the children too loose their innocence and move to adulthood, as they gradually begin to develop a social conscience and understand that society is not at all as simple a they believed it to once be.