Even though Atticus lost the trial, it is clear that he proved Tom to be innocent of the beating and rape. He also showed that Bob is most likely guilt of abusing his children physically, and that he was responsible for Mayella's injuries. The reader gets the idea that Bob was angry with Mayella for coming on to Tom, and beat her after Tom fled home. Bob is angry that Atticus has shown this side of him to the town. By accusing Tom, Bob probably thought he would look like a hero, the protective father avenging his daughter. Atticus took that from him. In his angry and drunken state, he threatens Atticus and others involved in the trial. Because of his behavior towards the judge and Helen Robinson, we assume that his threats are real.