
let me be clear here, i am not taking sides with Seunghoo (i am a total Nakyumwhore and a Nakyum Defender). I just wanna give his character the benefit of the doubt and to also give respect for author-nim since some people here badmouth her due to Seunghoo's character.
When the old servant said that Seunghoo was not like he was today, that gave me a hint that maybe he was deprived from genuine affection from people around him. And through Nakyum, who was affectionate with Inhun, that deprivation sparked a fuse to plunder the said affection all for him. He is a twisted guy with a whole lot of twisted ideals but lets consider the possibilities to why he became like that (i believe that's what the author wants us to ponder about).

actually I'm thinking the same thing. there's probably a betrayal of trust somewhere in his past, plus he's a noble and he regard highly of himself explains that high and mighty attitude. Aside from curiosity of how could he paint something that erotic I'm pretty sure what draws him to Nakyum is because his different from others.

That and he's someone who doesn't know how to relinquish power to someone else. His selfishness clouds the way he sees Nakyum and ruined any chance of there ever being real affection there. I mean NaKyum is attracted to him, imagine if he were to have wooed NaKyum instead of objectify him? He'd probably win him over damn I feel sorry for him too.
Also it was rape no need to put it in quotations. Let's not beat around the bush. Great comment tho, very well thought out theory.
since you guys are giving your cents about this intriguing story, I would also like to share my thought.
I wanna share my thoughts about Yoon Seunghoo.
Seunghoo is totally a "Nakyum Fanboi". We all know he had this interest in Nak because he was good and known in making explicit paintings. Thats just the first. I thought what really made him obsess over Nak is due to the reason that a subject of him (Nakyum) isnt unlike any of his other subjects who drools over and craves his attention. Nakyum is different, he is something he acquired by force (remember how he got Nakyum in the first place?).
He intend to keep Nakyum. Yes, we saw how he almost punished Nakyum by means of the beating but didnt do much damage to Nakyum because, according to him, he doesn't want Nakyum's work to be affected. BUT, subconsciously, he just doesnt want Nakyum to draw more distance from him.
The arrival of Inhun just made things hop onto another level. Now that Nak's attention is divided with most of it favoring Inhun, Seunghoo felt casted out. Maybe he realizes that what he is to Nakyum is a fanatic to his paintings. Knowing how Nak's admiration focuses to Inhun made him take advantage of Nakyum's drunken state and the delusion the liqour brought. I dont know why but I got the feeling that while they were doing that drunk sex scenes, Seunghoo's interest on Nakyum intensified. He may just want Nakyum to not leave and continue painting for him at the beginning, but now, he wants to monopolize the entirety of Nak. That resulted to chapter 25 or 26 where he "raped" Nak.
But why did he do it? My thought of it is because he wants to do it with Nakyum knowing its him and not Inhun. And those things happen and now he realized he doesn't bear any importance to Nakyum. That raged him and I do guess that, as to what the last frames of chap 26 is hinting, he will threaten to kill Inhun to subjugate Nakyum into having sex with him and to get the majority of his attention for him.
-- Lastly,
When everybody's feeling angry towards Seunghoo, here I am pitying him. although he is wealthy, known, intelligent, handsome and a sex expert, what I see is a character who craves and is a sucker for genuine affection, admiration and honesty.