There are hints in earlier chapters that Shuu reads Fist of the North Star (Raoh drawing) and Dragonball (asking for missing v18). He would prefer to be a hero like in his mangas, but instead all his dad and friends do is kill & rape innocent people. They also just killed Yoko, a woman he was getting emotionally attached to as a mother figure. In his mind he is doing the heroic thing by stopping villains, even if the villains are themselves.