What circumstances prompt Atticus to tell the children it is a sin to kill a mockingbird?

Atticus had given Jem and Scout air rifles but warns them not to kill a mockingbird because they only try to do good in the world. This theme of innocence and goodness extends to characters like Boo Radley and Tom Robinson.

thanks aslan ur the best god bless :)

Awwwwwwww. You're making me cry Jackie! You're too kind.