From Betrayed Hero to Invincible Demon King

Saikyou Yuusha wa Oharai Hako → Maou ni Nattara Zutto Ore no Musou Return

Despite being called ugly and detested for his looks, Galius was the hero who lived for the sake of mankind. However, the humans betrayed Galius. So, he too, forsook the humans. Then, after experiencing the warmth of the demon kind, he decides to help the demons in building their paradise!!! By using the strongest power given to him by God, this is the ultimate isekai fantasy of a man - the Hero - who becomes the strongest, unconquerable Demon King!

(Source: MANGA UP!)