According to legend, every 100 years the gods choose 6 people to protect the world from all that is evil. It is said that someone of the last group killed a demon. As you already know, the gods don't allow those chosen to kill, and that those who are chosen die if they kill. But you also know that when someone chosen is first killed, they gain powers. This is what happened, but since what he died from was killing a demon, he gained unbelievable power, and went insane.