Atticus bought Jem and Scout new air guns. When uncle Jack taught them how to shoot with the guns, Atticus said to Jem:
I’d rather you shot at tin cans in the back yard, but I know you’ll go after birds. Shoot all the bluejays you want, if you can hit ’em, but remember it’s a sin to kill a mockingbird.

Later Miss Maudie explained to Scout, “Your father’s right,” she said.  “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.”

If you can learn only one thing in life, let it be this.
Be kind to everyone, especially to those who can’t defend for themselves.