In chapter 25 of To Kill a Mockingbird, why was Tom "a dead man the minute Mayella Ewell opened her mouth and screamed"?

Tom Robinson was considered dead as soon as Mayella screamed simply because he was black and she was white. During that time period in the South, no one would have believed a black person over a white person regardless of the situation. Blacks were considered guilty if accused by a white before even getting a chance to prove their innocence. Atticus was an excellent lawyer and he proved Tom's innocence to everyone in the courtroom, but 'in the secret courts of men's hearts' everyone had already convicted Tom without a trial.