Because Atticus, the intelligent and calm lawyer and father of Scout/Jem, worsened his name. He embarrassed Mr. Ewell in an astonishing way that only ruined is reputation even more. Additionally, everyone in the courtroom knew what really happened after Atticus provided a significant deal of evidence towards how he abuses Mayella physically and sexually. He swears that he will get revenge, and later does by illegally entering the judge's home and he attempts to murder both Scout and Jem after the Halloween pageant. Luckily, Boo Radley or Arthur saves their lives.
