In chapter thirty of To Kill a Mockingbird discuss Scout's comment about the mockingbird in relation to Boo Radley.
Heck Tate spends much of the chapter trying to convince Atticus that it is not Jem who hurt Bob Ewell. By the time Atticus is willing to accept it, they come to the conclusion that Boo Radley killed Bob Ewell out of an effort to defend the children. Boo did a good thing to protect them. Although, the law would have it that killing is wrong. Atticus is ruled so often by the law that it is a struggle for him at times to separate the different types of right things to do in this world. The men agree to let the situation be an accident and keep Boo Radley's name or mention out of it.
They expect Scout to understand and do the same. She claims understanding by likening the situation to shooting a mockingbird. Boo Radley was an innocent man who didn't do anything wrong and deserves the treatment of innocence, just like mockingbirds.