Like the mockingbirds Atticus tells Jem not to shoot, both Boo and Tom are gentle and harmless. They do not hurt or destroy. In their own ways, both men bring goodness and joy into the lives of others. Also like the mockingbird, both men are defenseless against forces that would destroy them.
Boo Radley has been victimized since his youth by his family and his community. Effectively imprisoned in his own home, Boo has been made the monster in Maycomb. Despite these injustices, however, Boo has remained a good man. He takes pleasure in watching the children at play, gives them gifts that fascinate and delight them, and grows very protective of them. He saves Jem's and Scout's lives, at the risk of his own. Boo's heroism is kept quiet so that he will not have to endure public scrutiny. Scout, wise beyond her years. realizes that exposing Boo would be like killing a mockingbird.
Tom is a husband and father who takes care of his family and feels genuine compassion for Mayella Ewell. Tom understands Mayella's loneliness and miserable life, and he treats her with kindness. In return, he is arrested for a crime he did not commit, tried, convicted, imprisoned, and then shot to death. Atticus had told his children it was a sin to kill a mockingbird; Tom's destruction is sinful in every way.