The TV series. I don't see what was wrong with it myself. IF your friend suddenly betrayed you and tried to destroy the world, I highly dought the shock of that will ever be go away. And it wasn't that Kamui felt he was weak, the person he was fighting was his friend, and he believed that Fuma had the strength to overcome this power that turned him into the dragon of Earth, and in the end Kamui was right. Through his sacrifice Fuma was able to regain his true self, and thus they defeated teh Dragons of earth.