Yeah, how could a being who has always been able to "rewind" everything whenever he felt like it, ever get the chance to learn what "regret" is... He reminds me of Akura-ou/Kirihito in "Kamisama Kiss": guy was a yokai with perfect self-regeneration, so he never understood what pain and regret felt for everyone else until he became human
Exactly. He's a God, everything is under his control and his existence is connected with "forever". How could he ever understand the mortal experience? Most human principles, emotions, MORALS are all connected to mortality and to understand mortality, you have to experience it in a way that won't give you back the chance to become immortal again, nor gain back the complete control that was always part of who you are.
With that said, he simply doesn't fit with humans, nor is he capable of truthfully loving one. To love something completely different from you, you have to respect its existence along with its way of living. By our standards, his actions are nothing but a violation of the very things that makes us human.
How can a cosmic being understand regret? A cosmic being cannot suffer the consequences of his on faults so he cannot understand regret. Regret is for us, that live in a linear time, but a being that can change space time continuum by snapping fingers, alter memories, etc. has no concept of regrets as he doesn't need to put effort to fix his mistakes.

First of all, these are second hand spoilers i've heard from someone who's actually read the Korean novel. The story ends with them technically together, but only because he eventually finds a way to erase every single horrific thing he's ever done to her from her mind. She never forgives him, because how the freak could she? He's utterly irredeemable. The only reason why she stays is ignorance, no sane person would willingly exist beside something like him.
As we all know, he isn't human. He can't comprehend the value of morality. He's a cosmic being, written WAY too well as one. Once the side stories get adapted, you'll understand that what we've seen so far is nothing compared to what's to come. The things he's done to her, over and over again, across countless dimensions, make the current chapters look merciful.
She has killed herself more times than anyone can count. And every time, he brings her back. Not even death is an escape because motherfucker won't allow it. Which, in a horrifying way, makes sense since he's literally the creator of everything.
He tortures her simply because he enjoys it and because he doesn't understand that what brings him pleasure could cause her pain. Morality, empathy and compassio are nonexistent to him, he's a chaotic creature that no human could ever willingly love.
He makes her forget because even with all his omnipotence, he still can't change her feelings. The revulsion she feels for him is instinctive because the human brain has adapted to remember and 'feel' danger. Also it's basically one of the things that are unalterable on her.
So yeah, he gets what he wants because he believes (and technically does in his world with no morals) he owns her since he created her and the world. And there's absolutely nothing she can do about it. It could be you, it could be me and we would all act the same way as mc. Helpless as fuck.
Calling him a black flag doesn't even come close lmao
This creature needs a flag of his own. The horror tag is very well placed. As for the romance tag, it probably refers to the fake reality he creates for her (him).
Edit: I'm just glad the author didn't make her magically fall in love with a monster that destroyed her life. I'm also glad he is written that way. He's not human so of course he'd never act like one. Nor would he be compassionate towards the random things he has created. So yeah, i still love the story as horror, it's masterfully written.