In "To Kill a Mockingbird," Atticus Finch "takes aim" at "Maycomb's usual disease." He attempts to shake up the jury enough that they will realize that they cannot find Tom guilty simply because of what Mayella Ewell claims despite her being white and Tom's being black.
After Atticus shoots the mad dog, Miss Maudie tells the children, "If your father's anything, he's civilized in his heart." Indeed, this is true in the courtroom of Maycomb. Civilized, Atticus Finch essays to influence the jury to be the same. He attempts to get the jury to examine closely the testimony and the evidence in a totally rational and dispassionate manner. When he does this and displays his integrity in such an attempt at honesty, the blacks in the balcony stand in respect as Atticus Finch leaves the courtroom. To them, Atticus is a hero because he has fought the Jim Crow laws and the hatred in the hearts of some of the citizens of Maycomb.
He is a hero as he killed the mad dog, and did it in one shot: "Miss and you'll go straight into the Radley house!". He did it before the dog could get to any townspeople and spread the disease.
So... how does atticus's taking aim in the mad dog scene parallell that of when he takes aim during the trial?
When shooting the mad dog, he was the only one skilled enough to do the job. When taking on the trial, he is the only lawyer skilled enough and not blinded by prejudice to defend Tom Robinson.
He is the only one who dares to do both jobs. Even when put in he front lines of danger, he does them. And does them welll. He is therefore a hero. He puts himself in front of others.