500
                            “'Shoot all the blue jays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird'" (Atticus). That was the only time I ever heard Atticus say it was a sin to do something."
This is the title of the story and Atticus mentions to them specifically not to kill mockingbirds.  Why is it a sin to kill a mockingbird?
                            What is Because they only make music for us to hear and do not hurt/bother anyone