The quality that Lee stresses most about these two men is their innocence. Both suffer abjectly in prejudice-riddled Maycomb. Tom Robinson is, of course, the more dramatic example of this, as he is wrongly convicted of rape and dies in a desperate attempt to escape. He is judged by his race, not by his actions. What we see of him at the trial, he appears humble, respectful, and kind. Yet the jury are simply not able to see past his colour. He does not stand a chance, even in a court. Lee makes it quite clear, particularly through the reactions of the unbiased Finch family, that this is a shameful case.
Boo Radley is no less innocent. Like Tom, he is a kind, quiet person, who is so unable to deal with the harsh realities of the world that he simply retreats from it. Because of his reclusive ways, he is also unfairly judged in Maycomb; he functions as a kind of bogeyman for the children at first, because that is how the adults generally regard him, as being odd and therefore someone to be wholly wary of. Yet, in reality, he is timid, gentle and thoughtful, as shown in his many acts of kindness towards the children, culminating in his rescuing them from the depraved Bob Ewell. It seems that children are the only people he can really be at ease with, as he can relate to their young innocence. It is through Boo Radley most of all that Scout learns the vital lesson of understanding others; she comes finally to see and value him for the decent human being that he really is, instead of fearing him.
Lee makes us feel great sympathy for both Boo Radley and Tom Robinson, and also indignation at the way they are treated by much of society. They are both linked to the symbol of the mockingbird, which represents innocence, and which of course also figures in the title of the story. Miss Maudie, agreeing with Atticus that ‘it’s a sin to kill a mockingbird’, explains why:
Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird.(chapter 10)
Mockingbirds, then, are seen to be entirely good-hearted and harmless; they try only to help others. Tom Robinson and Arthur Radley both fit this description, but society makes them suffer. Therefore, we feel compassion for them.
Thanks a lot - very helpful.