When Atticus buys his children air rifles for Christmas, he tells them "Shoot all the blue jays you want, if you can hit 'em, but remember, its a sin to kill a mockingbird." The quote from Atticus was considered the highlight of the book, and the title is after all, "To Kill a Mockingbird". It likens Scout's trials to the "sin" of killing a Mockingbird. Hope this helps.
