The premise is interesting, but the plot doesn't suit it. I think this would have been a whole lot better if it was dark fantasy with some comedy. For example:
After being betrayed by his comrades (instead of one) when killing the current demon lord, the hero, Fried, is killed. The demon lord on the verge of dying sees this and laughs about how sarcastic this scene is, revealing that she was also a human who was betrayed by her comrades. She tells the hero that it is his fate to become consumed by the same hatred that consumed her and starts the resurrection process. When Fried next awakens, he now has the Demon Lord's crest and horns on his head. Seeing how the humans are the actual monsters, he decides to protect the demons and get revenge.
Or something like that. 😉