Atticus does an excellent job defending Tom Robinson. He points out the fact that Mayella was beaten by someone who is left handed. He does this based on the side of her face that is beaten. Atticus proves that the one who assaulted Mayella was left handed. Then Atticus points out that Tom Robinson could not have beaten Mayella because his left arm is crippled.
While this should have been enough evidence to find Tom Robinson not guilty, the all white jury finds Tom Robinson guilty. Of course, they find him guilty because he is black. The all white jury condemns Tom Robinson and dismisses evidence that proves Tom Robinson is innocent.
Atticus points out that Bob Ewell, Mayella's father is left handed. This evidence proves Bob Ewell could have beaten Mayella:
Bob Ewell is despised by Maycomb society as a shiftless drunkard. He is unable to keep a job, spends all his relief money on alcohol, and traps animals outside of hunting season. He provides little support to his large, motherless family, and is reputed to beat his children (and perhaps sexually abuse them too, as Mayella's testimony hints). Angered and shamed by his exposure on the witness stand, Ewell makes threats to Atticus and others involved in the trial, but never risks direct confrontation.
The all white jury does not take evidence that Bob Ewell could have attacked his own daughter into consideration. They are not about to convict a white man over a black man, even though they despise the kind of man Bob Ewell is. This shows how racist Maycomb is.
Tom Robinson is found guilty of assaulting Mayella while Bob Ewell walks away a free man.