What does Tim Johnson in To Kill a Mockingbird symbolize?

Tim Johnson, the dog, could symbolize the prejudice and mob mentality of Maycomb. For example, because most of the people of Maycomb are guilty of hateful prejudice, when they get together, they act like a rabid dog.  They are out of control, take action or speak without logic, and strike without warning.

Tim Johnson also represents the town's prejudice in the sense that Atticus seems to be the only Maycomb resident who can "save" the town and his children from the disease of prejudice.  The sheriff calls upon Atticus to kill Tim Johnson because he doesn't feel qualified to do so.  Likewise, he warns Atticus when the mob is coming for Tom Robinson; so that Atticus can go protect Tom.  Most significantly, Atticus chooses to defend Tom Robinson because it needs to be done, and he is not one to shirk his duties. The same is true of his shooting Tim Johnson--it is his duty to protect his children, whether it be from prejudice or a mad dog.