Mockingbirds don't do one thing but make music for us to enjoy.
They don't eat up 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 mockingbird.
In this part of the book Jem and Scout get airguns for Christmas. Atticus tells them that it is a sin to kill a mockingbird and Ms. Maudie Atkinson expands on that.
To my way of thinkin' Mr. Finch, taking the one man who's done you and this town a great service,
What Ms. Maudie says demonstrates to Scout and Jem that because mockingbirds do nothing but good that it is evil to kill them.
an' draggin' him and his shy ways into the limelight - to me, that's a sin.
Although it may not be apparent to Jem and Scout at first, later on in the book during the next sequence Scout realizes the meaning of Miss Maudies words.
It's a sin and I'm not about to have it on my head.
This part of the book takes place immediately after Mr. Ewell attacks Jem and Scout. Boo Radley ends up saving the two but killing Mr. Ewell in the process.
Sheriff Heck Tate gives a message with the same meaning as that of Miss Maudies. The difference is that this is dealing with a person and not just birds.
After Sheriff Heck says his two cents and his good nights, Scout realizes the meaning of Miss Maudies and Mr. Tates words and draws a comparison between the two stating that "Well, it'd sort of be like shootin' a mockingbird, wouldn't it?"