In To Kill a Mockingbird, why does Harper Lee include the scene where Atticus shoots the dog?

There are both practical and symbolic purposes for including the incident where Atticus shoots the rabid dog.  First, the incident shows that the town of Maycomb looks to Atticus to do what most people can’t.  Second, it shows that Atticus is able to surprise his children.  The mad dog incident is also symbolic. Atticus defends the town for the first time in this chapter.  At this point, it is Atticus versus rabies.  During the Tom Robinson trial, it is Atticus versus racism.  Maycomb relies on Atticus to do its dirty work.

Scout is surprised to learn that her father has hidden talents.  She describes him as “feeble” and sees him as old.  Atticus isn’t interested in guns.  He doesn’t even teach his children to shoot their air-rifles.  That task falls to Uncle Jack.  Atticus also warns his kids to be careful with the guns.

Atticus said to Jem one day, “I’d rather you shot at tin cans in the back yard, but I know you’ll go after birds. Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird.” (Ch. 10)

Imagine their surprise when they find out that their gun-avoiding father is actually One-Shot Finch!  When a rabid dog threatens the neighborhood, Atticus is called to deal with it.  The Sherriff, Heck Tate, doesn’t believe that he can shoot the dog as efficiently as Atticus.

“For God’s sake, Mr. Finch, look where he is! Miss and you’ll go straight into the Radley house! I can’t shoot that well and you know it!”

“I haven’t shot a gun in thirty years—”

Mr. Tate almost threw the rifle at Atticus. “I’d feel mighty comfortable if you did now,” he said. (Ch. 10)

Atticus is able to do what no one else, even the sheriff, is able to do.  He shoots the mad dog and saves the town from the potential danger.  A rabid dog can be very destructive.  Rabies can kill a person as well as a dog. 

Lee includes this incident to show how essential Atticus Finch is to the town of Maycomb, but it is also foreshadowing of the role that he is playing with Tom Robinson’s trial.  Just as with the mad dog, Atticus will face down the threat—racism—when no one else can or will.  In that case, he will not so openly and easily defeat it.  However, he does make progress, and helps Maycomb understand that change will be necessary.