Rinjoku no Shiro Kairai no Ou

There is a kingdom where the king takes good care of people. Seemingly, it looks peaceful. However, there is a castle owned by evils floating above them. They are connected with each other through a gate and they can come and go freely. The ones who go through the gate usually carry weapons, so a terrible battle often takes place. The main character is a soldier. He joins a battle against evils and enters the castle. But there is no one inside, just the devil appears. He finally kills the devil and happily comes back to his troop. However, other soldiers see him and say, "Oh, the devil! We'll kill you!!" He doesn't know what is going on, but he defeats them and runs away.... A while later, a girl appears in front of him and says, "You are the devil now. Attack people and rape girls to keep on living." He has been transformed into the devil....