Why is the third brother in the TV series "My Child, My Home" in prison?

The third brother in the TV series My Child My Home was in prison because he had committed a serious crime in the process of committing a crime and was sentenced to a long term imprisonment. To be specific, Third Brother committed murder in the TV series and was sentenced to prison because of the complicated plot. In prison, Third Brother established a deep friendship with his fellow prisoners and finally achieved self-redemption through continuous efforts to transform himself.