some notable things to mention seunghyun is literally straight or was at some point which is why he rejected yuta back then plus he was only ever in love with that person at the wedding that got married but whatever. the main reason why yuta is being so nice to him rn like in the last few chapters, is because he wants seunghyun to let his gaurd down so yuta can get seunghyun to trust him then yuta can betray him just like how seunghyun did to him IDK CHAT THIS IS SO MUCH Yap but it does make sense its just weirdly like spaced in the story
This is my interpretation as well
The official translations might help because these last few chapters seem critical To the plot. I think Yuta also feels betrayed because the seme did rescue him at one point but then took him back to the brothel? Gave him money to study so maybe yuta thought seme would come back for him.

this is my general understanding of the plot for those asking:
from the beginning:
yuta (the uke) was/is essentially a sex slave kinda like the og uke in Pearl Boy, HE HAS been THROUGH HELL anyways he meets the seme during that flashback and the seme is really kind to him and refuses to have sex with him, yuta is absolutely in love with the seme during that time because of how well he treats him in that environment, the seme kinda eventually gives up on yuta and thus yuta gets married (which leads into the nobel plot that was mentioned). Yuta married an older man so the older guy dies leaving yuta to inherit the fortune and be connected to all those elders we keep seeing, but he sets his sights on finding the seme again for revenege but also because he still loves him. Yuta tells the seme he will give any amount of money (basically the casino) in exchange for his love and keeping him in captivity just like how yuta was when the seme left him.