Tom Robinson is an innocent black man who was wrongfully convicted by a racist all-white jury.
A major subplot of the story follows the trial of Tom Robinson, the man accused of raping Mayella Ewell. Based on Lee’s portrayal of him, he was an innocent victim. Robinson is a nice man, and he felt sorry for Mayella because of the way her father treated her.
He knew she was alone with too many children and no one to help her, so he tried to make her burden lighter by assisting her when she asked for help with chores. It was not because she paid him, but because he knew she was lonely and miserable. For this, he was convicted.
Lee portrays Robinson as a family man and a good man. He helped Mayella on many occasions. Her advances to him were unreturned. He was just helping her because he pitied her.
Mr. Gilmer smiled grimly at the jury. "You're a mighty good fellow, it seems- did all this for not one penny?"
"Yes, suh. I felt right sorry for her, she seemed to try more'n the rest of 'em-" (Ch. 19)
Of course, the white jury does not like hearing that a white woman kissed a black man. They like hearing even less that a black man felt sorry for a white woman. Such things break the color boundaries. Blacks are supposed to know their place. These are the reasons Tom Robinson was convicted.
Atticus describes the Robinsons as “clean-living folks” (Ch. 9). He defends Tom Robinson because he is ordered to, and because his conscience tells him that it is the right thing to do, even though he knows he will lose the case. Cases of race are unwinnable, because almost everyone in Maycomb is a racist.
The only thing we've got is a black man's word against the Ewells'. The evidence boils down to you-did-I-didn't. The jury couldn't possibly be expected to take Tom Robinson's word against the Ewells. (Ch. 9)
When Tom is on trial, his wife is unemployable, even though he did nothing wrong, because "Folks aren't anxious to- to have anything to do with any of his family." (Ch. 12) They have to take up a collection for the family in church just so she can get by. Tom’s life is ruined, and his family is ruined, just because of something a scared girl did and said.
Atticus proves at the trial that Tom Robinson physically could not commit the crime because he has one bad arm, and in fact that Mayella’s father is the one who beat her up. The case against Tom seems pretty weak. However, Tom is convicted based on race alone. Atticus is not surprised. He expected this. He tells Tom they have a chance on appeal.
When we talk about killing mockingbirds, Tom Robinson is one of the mockingbirds being killed. He is an innocent man who does nothing but mind his own business and help a girl. For this he is put on trial, convicted, and sent to prison. Knowing he will never be overturned on appeal, he decides to commit suicide attempting to escape, and is shot. His is a sad story, but the inevitable truth of racism.