The rabid dog is highly representative of what is going on in Maycomb. Just as the dog is crazy, the town is crazy as well. The citizens of Maycomb cannot see that racism is something that must be fought rather than not fostered. The madness is so great that Tom Robinson is not given a fair trial, when all evidence points to his innocence. Moreover, the disease of racism seems to be growing.
That Atticus is the one who Heck Tate asks to put the dog down is also symbolic. Atticus is the one who can cure or at least curb the madness. He is probably the only one. Moreover, the fact that Heck, the law in town, cannot do this shows that to change the opinions of people requires more than the law. Here is what the text says:
“Take him, Mr. Finch.” Mr. Tate handed the rifle to Atticus; Jem and I nearly fainted.
“Don’t waste time, Heck,” said Atticus. “Go on.” “Mr. Finch, this is a one-shot job.”
Atticus shook his head vehemently: “Don’t just stand there, Heck! He won’t wait all day for you—”
“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!”
Atticus is a protector of the town by his honesty, integrity, and courage.