What an interesting question. I have never thought about associating Tim Johnson, the rabid dog that Atticus shoots, as a symbol of racism. I have always thought the dog was used as a vehicle to teach the children that their father had many talents that they did not know about, shooting being one of them, and that, as Miss Maudie says:
People in their right minds never take pride in their talents.
That said, however, perhaps Tim Johnson staggering down the deserted street of Maycomb can symbolize the Southern racism that is also staggering down the street, on its way to its death. This story was written during the Civil Rights movement, so perhaps the exposure of racism by the media and the focus given to it on TV was the beginning of the end for the Jim Crow brand of racism that was still widespread throughout the South. Perhaps Tim Johnson's sickness could be compared to the sickness of the racists in the town of Maycomb. Since Atticus killed the dog, perhaps this can be symbolic of the fact that his defense of Tom was also going to be a bullet in the back of racism.
I don't know...........perhaps this is stretching it a bit, but.........what do you think?
A quote you might use:
If your father's anything, he's civilized in his heart. Marksman ship's a gift of God, a talent --- oh, you have to practice to make it perfect, but shootin's different from playing the piano or the like. I think maybe he put his gun down when he realized that God had given him an unfair advantage over most living things. I guess he decided he wouldn't shoot till he had to, and he had to today.
With regard to the quote -- Atticus realizes he must use his talent now to kill the rabid dog, as it is a danger. He soon will realize he needs to use his talent as a lawyer to do what he can to defend Tom Robinson, and move towards eradicating racism in Maycomb. Later in the novel, even though he loses the case, he tells Jem that his defense of Tom is at least a start towards fighting racism.