I Reincarnated in Another World and Somehow Became the Demon King

Ye Mantian, a shut-in who had grown disillusioned with the real world, lived alone and lay flat for three years before finally working up the courage to leave his room.

He rushed into a fire to save a little girl trapped inside. Without a protagonist's halo, he predictably died.

When he woke up again, he found he'd been reincarnated in another world. But this other world wasn't as wonderful as he'd imagined.