In chapter 10. Shoot all the blue jays you want, if you can hit 'em, but remember it's a sin to kill a mocking bird." What does it mean and how does it relate to the novel?

In addition to bearing the title of the novel, this passage demonstrates yet again how similar Atticus and Mrs. Maudie are. Both agree quite strongly that it is a sin to kill a mockingbird, an animal symbolic of Boo Radley and Tom Robinson, as neither has caused harm, and prove only to have pure hearts.