Harper Lee had Atticus explain an important principle to his children. When he gives them guns,he reminds them that they can shoot many things,but it is a sin for them to kill a Mocking bird.Basically he tells his children that a Mocking bird does nothing but play music for our enjoyment and that they do nothing but sing their hearts out for "us",as he puts it,and that that's pretty much why it's not okay to kill them.
