I'll admit I'm not satisfied with this ending at all, if they had extended this even a little further I would have been happy. Something along the lines of 'So I became the Demon King that declared peace' or some other suitably stereotypical line would have been fine. Ending it here just feels like a halfhearted leadup to sequel, or a revelation that this is actually a prequal to something else. I could have actually seen this as a cool spinoff to a series with an interesting Demon King character but since that series doesn't exist this just feels half hearted.