Okay, here goes! The Mockingbird is a symbol of beauty and innocence. The bird sings, lets us admire its beauty and flies away. Miss Maudie explains that,
"Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mocking bird."
There are a few characters that are symbolic of mockingbirds:
Tom Robinson- Tom is the large gentle Negro man who Mayella and her father accuse of rape. He only feels sorry for Mayella and for this he is driven to bolt from prison and get shot.
Boo Radley- Boo is Scout and Jem's guardian angel. He is an introverted recluse who, despite being imagined a monster, saves Scout and Jem from Bob Ewell. Heck Tate and Atticus decide to keep Boo a secret because the publicity would kill him.
It would be a sin to hurt both men because they are beautiful inside and, like the mockingbird, only wish to make people happy.