What is the symbolism of the mockingbird and Atticus saying, "It is a sin to kill a mockingbird." What does this represent to you?

Certainly Boo and Tom Robinson represent a mockingbird. Boo's mental and emotional state is fragile. He does kind things but his spirit and mental state can't handle attention.Boo needed to be spared the publicity of being a hero. Boo needed his privacy. Tom Robinson also does kind things and is punished for them. Both men have an emotional fragility about them. Like the mockingbird, it would be a sin to hurt them.