It is Scout that reminds Atticus that it is a sin to kill a mockingbird. The metaphor isn't lost on her father. Atticus has a very human philosophy towards the world. He believed that the greater good must always be recognized and caught for. He believed that doing the right thing is essential even if it goes against the common idea. Atticus realizes that Boo Radley is one of Maycomb's precious mockingbirds and the law, in this case, would be better kept in context. A publication of Boo's story would not do anyone, especially Boo, any good. He leo feels he owe the "malevolent phantom" at least that much.