I never thought I would say that

I mean, it's a a pretty realistic description of how some 'relationships' play out, both from the victims and the abusers POV. I do understand, that this probably isn't, what most people are looking for when reading or watching stuff.
Most of us do it, to escape from reality to somewhere, where everything is either fine/perfect or at least the villains always get what's coming to them in the end, when in reality it hardly ever tends to work out that way.
But the abuse starting gradually, with the abuser playing nice at first just to become worse and worse over time, and the fact that it isn't always (hardly ever) as simple as 'just leaving' to put an end to it, is pretty accurate to real life abuse cases.

You said pretty much everything I was going to reply. Many readers really act like character development means the character fights back or snaps out of manipulation, when it isn't that. Ut's slow, and many times abuse victims go back and ford. I even feel many readers have NO empathy for MC nor even care that he's not well in the head either because of all the pain inflicted on him. They want him to be the perfect victim that never gives up and HAS to fight back or else it's not character development

Honestly? The thing, that shocks me the most when reading the reactions to Non Zero Sum, are the many MANY disparaging remarks/comments towards Haewon.
Most of us would react more like him than any of the MCs, that actively fight back and/or take revenge against their abuser(s), if we were put in the same situation.
The lack of empathy towards an abuse victim, who did nothing wrong, is quite horrifying to me. No wonder, victim blaming is still so prevalent in our society.

Yes! It's crazy how there's people angry at him when it should be them getting angry at those guys. I'm pretty sure those people and mostly everyone would act the same. It's easy to say Oh no I wouldn't I'm not stupid, but they disregard the fact that Haewon has been abused through his life, not only these guys but his father too, and I genuinely don't know what else would I do in his position. They expect so much from him, but he's the victim. Why are you pissed at him not running away? To me it feels like when people act the same when women stay with their abusers, they don't know how terrifying that situation is, abusers manipulate and threaten the victims in such a horrid way that it damages them. They want Haewon to act like a person who hasn't been abused. Crazier when considering how much pain was shown that he endured, but those readers draw the line at an abused character smh. I feel the same way, you'd think people would be emphatic with him, but it shows as you said that society is fucked up.

They just don't like, how well NZS portrays the actual reality of abuse, I think.
MCs like Taekjoo (Codename Anastasia), Leewon (Roses and Champagne) or Taeui (Passion) also go through their fair share of abuse and rape, but since they are a) better equipped to handle it and b) it's fiction after all, they 'suck it up', get even stronger and successfully tame their (black flag) MLs, who ofc (because again, it's fiction), once tamed, never re offend again (at least not against the MCs).
Which is all nice and all, but not how it usually plays out in reality.
Abusers seldom change their ways, and if you try to leave the worst of them, they'll retaliate and even go as far as kill you for it (or at least try to).
Also, they break down your self worth so much, you don't even think you deserve better or that there will be someone else, who might actually love and accept you WITHOUT the abuse.
Just like Haewoon thinks, he needs Haeyoung because without him, he has no one and nothing else to turn to. Let's be honest, Haewoon has no money (or outside help) to even try and run away successfully. Haeyoung and the others would and could track him down, if he tried to run, and then things would just get even worse, than they already are.
He is stronger than I would probably be in his shoes, because he still wants to live (for now. I know, the suicide attempt is coming) despite his shitty situation.