Traveling Through a Different World, the Devil Is Actually Myself

Yes, I traveled through time and became a brave man from another world. As soon as I embarked on the adventure, I got my own invincible sword of courage. With this sword of bravery, I have the ability to become stronger as I fight. At the end of the journey, I walked step by step onto the empty demon king's throne. It turns out that the devil is myself! When did the problem occur? I am a brave man who wants to save the world! How could he become the devil! Who can explain it to me?