"Shoot all the bluejays you want, if you can hit 'em,"  Atticus Finch tells his children when they receive air rifles for Christmas, "but remember, it's a sin to kill a mockingbird."  With this simple directive from Scout's father, Harper Lee sets up the metaphor that she will thread skillfully throughout her novel.  Tom Robinson, the black man unjustly accused, will end up dead after being put in a hopeless situation by Maycomb's most disgusting and despicable resident, Bob Ewell.  Arthur Radley, nicknamed "Boo" by the children, lives in the literal sense, but his life was more or less taken from him in the figurative sense when his cruel and abusive father put him under virtual house arrest after a typical teenaged prank.  The "sin" of Robinson's death was that he died trying to escape prison when he never should have been accused, much less imprisoned, in the first place.  The "sin" of Radley' situation was the cruel, lifelong punishment the boy received for a silly prank in which all the other boys received mild punishments because no one was really hurt--except for Arthur.