In Harper Lee's To Kill a Mockingbird, the first thing that strikes the reader about Tom Robinson's time on the stand in court—during the trial in which Atticus defends him—is when he attempts to place his hand on the Bible to be sworn in.
In Chapter 19, Tom Robinson is called to the stand.
Tom Robinson reached around, ran his fingers under his left arm and lifted it. He guided his arm to the Bible and his rubber-like left hand sought contact with the black binding. As he raised his right hand, the useless one slipped off the Bible and hit the clerk's table. He was trying again when Judge Taylor growled, "That'll do, Tom."
Not only is reader aware that life must be difficult for this family man to support his wife and kids, but one must immediately sense that this injury would have made it impossible for him to carry out the attack as described by Mayella Ewell and her father.
We know that Tom went to jail for a time not because his crime was so terrible, but because he didn't have the money to pay his fine for getting into a fight—as the other man did.
Tom is a good worker. He is polite, tipping his hat to Mayella as he passes her home. The jury is told that Tom has done work for Mayella many times, without taking money to do so. This shows what a kind person he is. Tom notes he was happy to help her as her father did little to help around the house, and he knew they didn't have much money. Tom infers that Mayella was always taking care of the children who he could see through the windows of the house, watching.
When Mayella calls Tom into the house on this occasion, he asks about the absence of the children. She declares that she has saved for a year to collect enough money to send them to get ice cream. The quiet and Mayella's unusual behavior make Tom nervous. Tom is a black man in a white family's home. He knows his "place." Entering the house is not something he does lightly. It's almost as if he's too polite to say no.
Tom is worried, but I also see him as a gentleman. Mayella throws her arms around him, and while this is a fearful development for him, it seems that he hesitates to talk about it—almost as if it might be an embarrassment to her. As her advances continue, Tom (politely) tries to extract himself, but Mayella won't let go. Then Tom recalls hearing her father bellow from the outside—at Mayella...calling her a whore.
When Atticus asks why Tom ran, Tom said he was scared...
Mr. Finch, if you was a n***er like me, you'd be scared, too.
Tom is a victim of circumstance: chosen by Mayella because he was nice to her, and accused by Bob Ewell because of his anger and embarrassment over his daughter's (in his eyes) unforgivable behavior. Then, when Mr. Gilmer asks why Tom was always so ready to help Mayella out, Tom makes his biggest mistake—as a black man in a white courtroom in the Depression South: he says that he felt pity for her:
"...I felt right sorry for her, she seemed to try more'n the rest of 'em—"
"You felt sorry for her, you felt sorry for her?" Mr. Gilmer seemed ready to rise to the ceiling.
The witness realized his mistake and shifted uncomfortably in the chair. But the damage was done.
This is damning because the bigoted whites would find it appalling that a black man could ever elevate himself above a white person enough to look down and feel sorry for her/him.
Tom is the victim here—a tragic victim of Bob Ewell's racism and Mayella's abject loneliness.