Why according to Miss Maudie does Atticus believe it's a sin To Kill a Mockingbird?

She believes it is a sin To Kill a Mockingbird because they are signs of innocence: They do not do any damage to the community and they make the world a better place with their cheerful singing all day long. To kill a mockingbird, in that sense, would be taking an innocent life away from something that didn't deserve it. This relates also to Tom Robinson, a man condemmed to a crime that he didn't commit.