Boo Radley was a neighbour to the Finch's. He made friends with some boys when he was young and they committed some social vices like gambling etc. They crashed a flivver and were taken to court and Mr Radley, who was Boo’s father told the judge to look after Boo.
He couldn't live his normal life anymore and at a point he got really tired and stabbed his father with a scissors. Because he was always indoor, some members of the town council told his father that if he isn't removed from the courthouse basement, he will die of the cold and mold.
