Boo Radley and Tom Robinson share many similarities in spite of fact that one man is white and the other black. By juxtaposing these two characters, Lee proves that justice and compassion reach beyond the boundary of color and human prejudices. The novel's title is a metaphor for both men, each of whom is a mockingbird. In this case however, one mockingbird is shot, the other is forced to kill.
Boo and Tom are handicapped men. Lee hints that he may be physically unhealthy, and she makes statements that lead the reader to believe he may be mentally unstable. However, no character sheds any light on his actual condition, leaving the reader wondering whether Boo's family protects him or further handicaps him. Tom is physically handicapped, like a bird with a broken wing, but his race is probably a bigger "disability" in the Maycomb community. As a result of these handicaps, both men's lives are cut short. Whatever Boo's problems may be, the reader knows that something happened to Boo that has caused him to become a recluse. For all practical purposes, Tom's life ends when a white woman decides to accuse him of rape.
Boo sees Scout and Jem as his children, which is why he parts with things that are precious to him, why he mends Jem's pants and covers Scout with a blanket, and why he ultimately kills for them: "Boo's children needed him." Apparently his family disapproves of his affection for the children or Mr. Radley wouldn't have cemented the knothole. But Boo is undeterred and loves them, even with the probable knowledge that he is the object of their cruel, childish games. Tom also recognizes Mayella as a person in need. On the witness stand, he testifies that he gladly helped her because "'Mr. Ewell didn't seem to help her none, and neither did the chillun.'" Tom helps Mayella at great personal expense.
Both men know their town very well. Unbeknownst to the Finch children, Boo has watched them grow up. The reader can fairly assume that Boo is also familiar with the Ewells, and probably doesn't think much more of them than the rest of Maycomb. Boo and Tom have had minor skirmishes with the law, but that past doesn't tarnish the kindness they show to others in the story. The moment that Mayella makes a pass at Tom, he inherently knows that he's in serious danger. Truthfully, he probably knew that helping her without pay was not the safest thing for him to do, but the compassion of one human being for another won out over societal expectations.
The children treat Boo with as much prejudice as the town shows Tom Robinson. They assign characteristics to Boo without validation; they want to see Boo, not as their neighbor, but as a carnival-freak-show-type curiosity. Ironically, watching the injustice that Tom suffers helps the children understand why Boo may choose to be a recluse: "'it's because he wants to stay inside.'"