Sling Blade's main theme is the redemption of Karl's lost childhood. Karl Childer's overly religious parents believed he was a punishment from God. They severely abused him, treated him like an animal, and forced him to live in a shed in solitude. Everyone in town picked on him and called him names. He was seen as a "retard" or slower than others. He had little to call his own. His only possessions were a Bible and several books on Christmas and carpentry. Karl was taught the Bible as a child, but it wasn't really the Bible. The stories they told him were not in the Bible. His parents had made up their own stories and had led him astray on the rights and wrongs of life. .
When Karl was 12 years old interrupted what he believed to be the raping of his mother. He took charge and killed the man with a "sling blade". He than found out his mother had wanted it so he killed her too. When he killed his mother and her lover he thought he was doing the right thing. Karl believed that if you have to sacrifice to do the right thing than you should do it. Karl was locked away in a "nut house" for the next 25 years for rehabilitation and correction. .
After Karl's release he quickly made a new friend, Frank. The young boy accepted Karl with out question because he seemed to be more childlike than man. The two become soul mates. Karl related to Frank through his childlike manner as well as his parental setup. Frank's mother was a widow dating an alcoholic, abusive man, Doyle. Karl saw himself in Frank and decided to watch over him. The parallelism between the two characters was shown throughout the movie. The love they shared will save them. The "boys" faced their troubles on a simple level. They avoided the complications of adult views and judgments. .
Karl had read the Bible and believed he knew the difference between right and wrong, but was faced with the problem of protecting Frank from a painful future with Doyle.