Boo, or Arthur, Radley and Tom Robinson are both presented as being victims of society - certainly society as it is represented by the town of Maycomb. They are both seen to be different: Tom because of his colour and Arthur because of his reclusiveness, and in this kind of prejudice-ridden society, that makes them feared and despised, and more liable to be picked on. Both men are symbols of goodness, innocence and vulnerability in a divisive and sometimes violent society. In this way they are both likened to the mockingbird of the title, which functions as a symbol for innocence. Both men try to help others. However, Tom's efforts in trying to help Mayella ultimately cost him his life. Arthur similarly helps the children and eventually saves Scout and Jem by killing Ewell, but this time Atticus and Sherriff Tate refuse to let the whole business be dragged into the light and into the court, after seeing what happened to Tom. They will not let Arthur run the same risk by coming too much into contact with the insititutions and rules and prejudices of society. They realise that such innocence suffers when subjected to official scrutiny.
Both Boo Radley and Tom Robinson are innocent. Tom Robinson didn't actually rape Mayella Ewell, but due to the prejudice in society the jury believes Bob Ewell more than Tom Robinson. Boo Radley is also innocent. The rumors that the children, Jem and Scout, hear from society about Boo is not true. We can see this when Scout meets Boo Radley and realizes that Boo is actually a nice and quiet person. He is not as awful as the rumors say he is.