One character who exemplifies being indignant quite well is Bob Ewell. In the courtroom, Atticus pretty much revealed what a louse he was, and brought to light the fact that he probably beat his daughter, and who knows what else, and then blamed it on an innocent black man. It is shaming, and he doesn't like it. So, he indignantly starts making threats, spits on Atticus, spreads nasty comments about the Finch family, and in the end, is driven to his sordid attack on the two children. Mayella displays similar indignation in the courtroom; if it wasn't for that self-righteous anger, she might not have followed through with the accusation of Tom. These two characters and their indignation feed a social system that unfairly puts a man in jail.
Prejudice is a major theme throughout the novel, with many examples: the jury finding Tom guilty even though it was pretty clear he was innocent, the angry mob gathering outside the jail to hurt Tom, Mrs. Dubose's acrid comments against Atticus for defending a black man, and all of the derision Scout and Jem get at school and from family because of what Atticus is doing. Hatred is easily tied to any of these examples of indignity and prejudice. All of this is the wheel that keeps the social injustice in the novel going; without it, there wouldn't be much conflict, drama, or even thought-provoking profundity to it.