"To Kill a Mockingbird" is basically about coming to grips with the good and evil that coexist in people and in society.  How individuals deal with the discovery of evil - in this case, mindless, angry rascism - is the core theme.  It is no accident that the Finch family is named after a small songbird.  The father, Atticus has already come to grips with the presence of evil and has committed himself to fighting sin, while simultaneously seeking to have sympathy for the sinner.  Scout more or less follows in his footsteps, reaching an enlightened attitude toward evil, while Jem allows evil to create dissolution.  I believe that Scout actually compares Boo Radley to a mockingbird at somepoint in the story, which underscores the symbolism - Boo Radley is a misunderstood innocent.

BofP