The devs were greedy, terrible at their job, and lied to the consumer base about the ending, so of course they'd be universally panned, and the game abandoned. MC is just one of many who did just that. The character he created to play as, Erina, apparently had self-awareness, and once he abandoned the game (and by extension, her) she went completely batsh*t insane and mu*dered literally everyone and everything around her. Her goal was to become the new demon king and give her 'Master' a reason to return, but of course he didn't because that's not how that works. She also just straight up tortured the goddess who manages the game's system to forcibly bring MC into the world and reset and just pretend that she isn't actually the incarnation of pure evil