The testimony of Tom Robinson in Chapter 19 seems completely sincere and credible. It offers the best explanation of who beat up Mayella Ewella and why he did it. According to Tom, Mayella was blatantly attempting to seduce him. She grabbed him around both legs while he was standing on a chair. Then he is forced by the judge to answer Atticus' question, "What happened after that?" Tom says:
"Mr. Finch, I got down offa that chair an' turned around an' she sorta jumped on me."
"Jumped on you? Violently?"
"No suh, she--she hugged me. She hugged me round the waist."
"Then what did she do?"
"She reached up an' kissed me 'side of th' face. She says she never kissed a grown man before an' she might as well kiss a nigger. She says what her papa do to her don't count."
At about this point the girl's father yells at her through the window. According to Tom's temtimony:
"He says you god-damn whore, I'll kill ya.
Tom Robinson's obvious reluctance to divulge any of these details makes them all the more believable. Any impartial jury would acquit him of the charges against him, but the all-white jury is hardly impartial. Even though Bob Ewell and the whole Ewell family are regarded as "white trash," they are still favored over the black Tom Robinson.
It is obvious that Bob Ewell has been having an incestuous relationship with his daughter Mayella. It is also obvious that Mayella's testimony, which takes up most of Chapter 18, is unreliable. Her father must have beaten her and threatened her with more of the same punishment if she did not testify that Tom Robinson raped her. She has only had two or three years of schooling. She is cunning but not intelligent. Atticus' cross-examination exposes numerous contradictions in her story. It is also in Chapter 18 that the reader learns that Tom Robinson's left arm and left hand are virtually useless, so it would have been impossible for him to blacken her left eye with his right fist.
Since Bob Ewell was the only other person present, and since he was in a rage at what he saw through the window, it is apparent that he must have been the man who beat up Mayella.