Why is it a sin to kill a mockingbird in To Kill a Mockingbird?What is the larger principle involved here?

In Chapter 10, Atticus makes the following
statement: “Remember it’s a sin to kill a mockingbird.” Scout asks Miss Maudie her
opinion on this and the woman responds, "Your father’s right . . . Mockingbirds don’t do
one thing but make music for us to enjoy . . . but sing their hearts out for us. That’s
why it’s a sin to kill a mockingbird.”

The larger principle
suggests a theme of good versus evil--of good things, like mockingbirds who commit no
wrong, being destroyed by bad things. For instance, this can be applied to Boo Radley.
He is a "good" character--he helps the children, leaves them gifts, and so forth, and
yet because of his dark history they are frightened of him. Additionally, his abusive
past indicates the "evil" aspect of the theme.

The book
develops this further in later chapters, but basically it comes down to the issue of
protecting the weak--the characters (Atticus and Scout in particular) prove by word and
deed that it is necessary to protect those who cannot protect
themselves.