Because It's the Rule.

Kyosuke and Mariko live modestly but bravely. They are siblings alone, without parents. One day, the younger sister decides to get married, and happiness finally comes. Her partner is a rich young man. It was the best time for them. However, the younger sister's body is discovered. It's a rape and murder. Kyosuke is crushed by grief and doesn't know what to do. The police are investigating the matter, but time passes without finding the culprit. One day, when he stops by a rental video store, he finds a new rape video. When he rents it, he sees his younger sister being raped. From that moment, the prelude to revenge on the perpetrators who drove his younger sister to death begins