Why is it bad to shoot a mockingbird in to kill a mockingbird?

Its a sin to kill a mocking bird because they do not do any harm, they don't nest in corn stalks, ruin crops, cause problems; all they do is sing beautiful music for everyone to hear.

This can be compared to tom robinsons death, all he did was help people, did no harm, and was killed doing it.

Atticus states this in the movie, and i cant remember who says it in the book, either miss Maudie or atticus :\