There Are No Bad Heros In The World summary:

“A man once called the strongest in the world. The heavyweight champion of the world’s largest martial arts league. Choi Hyun-seok, who was once the mightiest man, begins a new life as a hero in another world after his death. ‘A hero who monopolizes wealth, fame, and beautiful women!’ Dreaming of a glorious future, he arrives in the other world, only to encounter a monstrous beast with a head one size larger than his own. The monster, a high-ranking officer of the Demon King’s army, asked: ‘Hero? Are you the so-called hero?’ ‘Uh… I guess I’m the hero…?’ And so, hell began.”