Yuusha Yukio

The story centers on Yukio, a hero who instantly defeats the Demon King and becomes a hero of his time. Yukio, who has everything he desires and considers himself "the happiest man in the world," gradually finds himself losing his sense of joy in everyday life. Despite having everything, Yukio begins to see ordinary people walking the streets as happier than he is. Unable to understand why, he agonizes over the matter and eventually takes a drastic action.

(Source: Shueisha, translated)