Scout remembers that memorable morning in February when Atticus shot the rabid dog, Tim Johnson, that was coming up the street. She remembers it so vividly that the hot packed courtroom seems to dissolve into that same empty waiting street on a cold winter morning, and Atticus and Sheriff Tate to her watching eyes appear to take on the roles that they enacted in that particular scene, when Mr Tate authorized Atticus to shoot the dog by saying ‘Take him, Mr Finch.’ But this time, Atticus’s authority does not prevail, and Tom Robinson, the man he fought to save, is condemned.
...it was like watching Atticus walk into the street, raise a rifle to his shoulder and pull the trigger, but watching all the time knowing that the gun was empty. (chapter 21)
In this effective splicing together of two apparently unrelated incidents, Lee highlights the central theme of the book, which is prejudice, and more particularly racial prejudice. Whereas Atticus was well able to destroy a literal mad dog with just one shot, he is ultimately helpless when faced with the mad dog that is racism. Racism is presented here most effectively as a wild beast and a deadly disease in one. It is something that cannot be reasoned with; Atticus’s passionate and eloquent plea for justice in Tom Robinson’s case goes ignored by the all-white jury. His attempts to shoot down racism, at least in this particular instance, are unsuccessful.