I really don't understand the mc's logic. Having humanity unite against the demon lord will only change things on the surface. Nefarious people will still look after their own interests, even using the demon lord's existence as an excuse to do harm to others in the name of the greater good in the exact same way that they did with the hero. At the same time he is responsible for atrocities of the same scale if not much greater than the most influential evil powers in the world he is trying to wrest control from and he will continue to perform these atrocities in order to maintain his position as a threat to humanity. He's trying to trade one evil for another, but he is failing miserably and just compounding the evil.