So far, I interpreted the story the same way. Something I can't understand is how tf Seunghyun forgot about the promise he made to Yuta and needed 7 chapters to finally remember it and piece everything together. He doesn't seem like someone who makes empty promises and remembering that in chapter 1 he says that he's never let anyone get off the hook after breaking a promise to him, it all seems even more bizarre that he himself would break a promise (especially since the consequences of that would change Yuta's life for 10 years..) or maybe he's really just a hypocrite that doesn't follow his own rules...
Regarding revenge- I think that Yuta is partially getting closer to Seunghyun to protect himself. He is the head of the family, and the sole breadwinner (fucking leeches...) but he doesn't have a bodyguard and allows the elders and Tetsu to harass him (?) Maybe he hopes that having Seunghyun by his side would keep these bastards away? In the beginning of the story it looks like Yuta is some badass black flag mastermind but the more we learn about him and his life, the more it's clear that he is just trying to survive just like he did in the Golden Pavilion :<
so true!! but I have the theory that Seunghyun didn’t forget but he arrived late, I don’t know if you remember but in chapter 7 (or 8 I don’t remember well) he asks Yuta if he is doing all this to him because he forgot his promise and Yuta pretends that he doesn’t remember that, and they show a flashback of Seunghyun surrounded by dead people while he lights a cigarette, it looks like the scene where Dooshik went to save Jooha in those recordings and he fights with everyone, maybe it’s a comparison of how Seunghyun couldn’t do that, although I think that maybe I’m taking the theory too far lol, the truth is that they will possibly show us what happened between them at the end of the season T-T. But I like u theory that Yuta uses Seunghyun to protect himself more than to ruin everyone from his past, and knowing this author, it's very possible that this year of fake marriage will be enough for these two to fall madly in love with each other.
I remember that scene, it's brought it up twice- once, when Seunghyun arrives to GP for his promotion ceremony and the boys are gossiping that he had recently killed a whole gang because they were pissing him off (indicating that he's cruel and will be terrible to them too). Then this panel is shown again in ch 7, when Yuta says that he's always looking down on people because he doesn't know failure or humiliation (which is bullshit but Yuta obviously meets Seunghyun when he's successful and making moves in Yakuza so it makes sense to assume that he wouldn't understand someone in Yuta's position). I really like your theory that he was just too late, because not picking Yuta up when he promised to do so seems out of character for him and makes it difficult to make it a love story (even if it's enemies to lovers..).
As for ruining everyone, Yuta mentions that Seunghyun's punishment will be association with someone with Yuta's "background". He often mentions that he knows what Seunghyun really thinks of him, but it makes me wonder why Seunghyun hates prostitutes so much? In the flashbacks, he seems very empathetic and understands that Yuta didn't choose this life, yet even his character profile states that he hates prostitutes and hates Dooshik for being a sex worker in the past :<

Ok, I re-read this for the fourth time and I think I understand a bit, please leave your theories:
10 years ago, Seunhyun and Yuta met. Yuta didn't trust anyone in the yakuza industry and saw them all as evil, until he met Seunhyun, who, unlike the others, showed him pity and "care," even being a glimmer of hope that he could get out of that place. Now, we're given to understand through flashbacks and conversations that Seunhyun promised Yuta something and didn't keep it, causing to Yuta to shatter the image he had of him (possibly he promised to find him and get him out of that place). Now my theory: In these 10 years that passed, Yuta has formed the idea that the only way to save himself and get revenge is by doing everything himself. No one has EVER shown him respect, so situations like having s*x in front of old people are normal for him. His hatred for Seunhyun arises from the fact that he sees him as a hypocrite. He thinks that Seunhyun hides his true self in his small acts of nobility and his dignified and clean facade. He wants Seunhyun to hate him and finally show his real face, that he is the same as everyone else in that industry. Maybe that way he will stop feeling the small feeling of "love" that the kind memories that Seunhyun made in his past when he worked in the brothel generate in him, and finally be able to move forward with his revenge. Yuta's revenge on Seunhyun is basically condemning him to live with someone he truly hates, to torture him. But Yuta doesn't understand that Seunhyun feels a lot of pity for him (and even a little non-sexual desire) and he feels indebted and tries to help Yuta.