The case of Tom Robinson is decided by the prejudice of the jury, not by the guilt of Robinson. When Tom says that he, a black man accused of raping a white girl, actually feels sorry for the girl because of how bad her life is, it greatly offends the white jury members. Within their society, he was as low as it gets, yet he had the audacity to feel sorry for a white person. His comment riles the audience back up, and removes any sympathy they may have felt for him.
when tom robinson says he feel ssorry for mayella, he is simply a black man and cannot feel sorry for a white woman, which to the jury means he is lying and therefore everything prior to this he was "lying" so the jury finds him guilty.