"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

I agree, some evil must be fought not with sympathy but with might. But not to lose one's sense of joy and appreciation of the world is what makes Atticus so refreshing a character. Despite his confrontation with evil, he remains positive on people and looks to find goodness which he does.

My identification with the actor is that he lived life the same way.