What passages in Harper Lee's To Kill a Mockingbird tell us Tim Johnson, the mad dog, represents racism?

In Chapter 10 of Harper Lee's To Kill a Mockingbird, one reason why we know Tim Johnson, the rabid dog, symbolizes racism is the dog is shot to death by Atticus, one who normally refuses to shoot anything because he values all of life too much. Since Atticus is fighting racism by defending Tom Robinson, we know Atticus's killing of Tim Johnson also symbolizes Atticus's fight with and attempt to kill racism. Racism is a serious, life-threatening problem in Maycomb, just as it threatens Robinson's life and just as Tim Johnson threatens the lives of Maycomb's citizens by being rabid. We can consider Sheriff Tate's comments on the urgency of the situation, as he pleads for Atticus to take the shot, as symbolizing the urgent need to kill racism:

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! (Ch. 10)

Hence, the dog symbolizes the deadly sickness of racism and the urgent need to kill it, to put an end to it, which is what Atticus is trying to do through his defense of Robinson, just as he took on the responsibility of killing Tim Johnson.