From Mightiest Hero To Demon King (LN)

From Mightiest Hero To Demon King (LN) summary:

Garrius, the mightiest hero in all of Midterria, has vanquished the demon king. Perhaps his little heart-to-heart with that old lizard left him a bit unsure about whether he’s done the right thing, but the demons are wiped out. Humanity is saved at last. All is well. There’s just one small hitch. Garrius’ homecoming is kept secret. When he finally gets an audience with King Edgar, he finds out that credit for all of his triumphs will be handed to someone else. What’s more, the king needs to banish him from the kingdom. They’re giving him the boot. Rather than futilely fighting this harebrained plot, the war-weary Garrius heads off to see what kind of post-heroic life he can make for himself. While he’s at it, maybe he can answer those doubts he had about just how evil demonkind really is.