Final answer:
The 'shootin' a mockingbird' theme in 'To Kill a Mockingbird' represents the harm in targeting the innocent and defenseless, paralleling the need to protect the vulnerable Boo Radley to avoid committing a sin.
Step-by-step explanation:
The theme emphasized by the words 'shootin' a mockingbird' is one of innocence and the wrongness of harming something that epitomizes purity and does no harm, as mockingbirds do not eat crops or bother gardens but only sing for enjoyment. The comparison of exposing Boo Radley to a sin underlines the theme of protecting the vulnerable and innocent, much like the mockingbird. In 'To Kill a Mockingbird', exposing Boo would be likened to killing a mockingbird, because like the bird, Boo is an innocent who does no harm and actually performs quiet acts of kindness.