Tom's crippled state is more than just a plot device. It also serves as an emblem for his disadvantage in life as a black man. His arm was injured in a cotton gin, a machine used primarily by slaves, and later, poor black workers in the cotton fields. The legacy of slavery cripples Tom in court and in his everyday life, just as his actual injury is a constant burden for him.
Mayella's sad situation comes out more fully in Tom's testimony. Her short comment about, "what her pap do to her don't count" hints that her father probably abuses her, possibly sexually. Mayella is as lonely as the "mixed" children Jem spoke of earlier, as she belongs to neither black nor white circles.
The idea that a black person could feel sorry for a white person refutes all of Maycomb's social assumptions, making Tom's courthouse comment extremely provocative. By nature, black life is thought to be inferior to white life, making Tom's feelings towards Mayella subvert everything that the town's social fabric is based upon. As Jem explains in chapter 23, every class looks down upon the class below it - so black people, as the lowest class, should not feel pity for anyone.
Dill's feeling of illness during Mr. Gilmer's cross-examination shows his extreme sensitivity, as a young child, to the ugliness of society's prejudices and evil. Scout tries to see Mr. Gilmer's actions as part of the method of the job he is trying to do, following Atticus's advice to try to "get into a person's mind" in order to understand them better. However, it is indisputable that Mr. Gilmer does not behave as honourably as Atticus. Atticus speaks to all the witnesses with respect, while Mr. Gilmer demeans Tom in court, calling him "boy" and sneering at him. Dill's classic method of managing uncomfortable situations is to run away, and he does so here, fleeing the courtroom with Scout at his side.
In Chapter 20, Atticus appeals to the jury's sense of dignity, and in putting together the facts of the case, he stresses the simplicity of the evidence and shows that the facts point toward Tom's innocence. As later becomes apparent, Atticus doesn't really believe that the jury will set Tom free, even though he hopes they will, as evidenced by his final statement, under his breath, "In the name of God, believe him." All Atticus can hope for is to leave an impression upon the town by exposing the truth for all to see.
Atticus's treatment of Mayella reveals that though a victim of many cruelties, she has chosen to bring cruelty upon Tom, and must not be excused for this. As he points out, Mayella wants to protect herself by placing her guilt on Tom, knowing that her actions will bring about his death because the jury will believe her, a white woman, and not him, a black man. Thus, she manipulates the unfairness of her society toward her own ends.
Mr. Raymond, as Scout notes elsewhere, is a person of high enough social standing that he can act in very unorthodox ways and have his behaviour accepted not only because, as he says, he gives the people a "reason" with which to interpret his behaviour, but also through the usual expression, "it's just his way." The ability to be pardoned for certain eccentricities isn't allowed to people of all levels of society. Mr. Raymond owns a great deal of land and is a successful businessman. However, if an Ewell displayed similar behaviour, he or she would not be excused so easily.
By Chapter 21, Jem was sure that the trial would go in Tom's favour after all the evidence was revealed. Therefore, the pronouncement of guilt comes as a complete surprise to his naïve mind, and he feels physical pain upon hearing each jury-member's "guilty". Jem is psychologically wounded by the results of the trial, feeling that his previously good opinion of the people of Maycomb (and people in general) has been seriously marred. Jem's trust in the rationality of the people has been beset by the knowledge that people can act in irrationally evil ways. He finds