You're so right, I don't know why people are so against character development that was foreshadowed (he says he doesn't feel like he has a sadistic side when it comes to Nanjo in chap 15 and that he feels safe just smelling his scent) and that perfectly aligns and makes sense with his backstory as a child who never had a family and who is starved for love.
I totally agree with you. Mokyeon's twisted personality comes from all the things he went through. He did bad things, alright, but the story itself explains what's behind it and is not justifying his wrongdoings, he actually had to face and admit it, suffer about it, had to apologize too and somewhat "pay" for it by being beaten up AND from all the distance Nanjo put between them. He had to face the disgust and rage from his loved one...
Now, I genuinely hope they can overcome all the obstacles ahead together

I understand the frustration of some people toward the "change" in personality of Mokyeon, but let's be real: this mf just desperately wants to be loved, because he never was/never felt loved. His traumas ended up turning him into someone who thought he actually didn't care about anyone and anything, because if he was indifferent, nothing would hurt him at all. But now he found someone he couldn't help but love, so it is too much for him to handle... And all his emotions and feelings are just overflowing, he can't help it. And knowing he fucked up real good also contributed to the way he's acting right now, because he finally realized what he was feeling, and with it came the fear of being hated forever by Nanjo. So I actually don't mind seeing him being real to himself, crying, totally helpless and acting all pathetic (apart of being suicidal, of course. That's another thing... He needs help). That's just my opinion, though. All that said, please, just fuck already, you two