
Yeah, that would be nice to see him say that, but it'd be a too great thing for him to do at this point, like you say; him flipping from his usual demeanor.
When he first started to respond to NK's kiss (destined to In Hun), the Lord was only aiming to give satisfaction to his attraction, lust for Nakyum and sleep with him. While they were doing it, he finally become more gentle and is acting like a lover (in order to fullfil NK's delusions)...
He's focused on pleasuring him, but is also on the listener side; passive side and just listens to NK. We can see mixed feelings occuring in his mind...

You're just bat-shit cray-cray, lmao. But can't blame you. Who isn't right now?
Those words aren't Seungho's. If you notice the bubbles pattern, when Seungho is speaking, it's round, neatly drawn, in reference to his deep (maybe? I like my seme with deep voice, lol) and steady voice. Nakyum's bubbles are all clumsy, emphasizing that he's overwhelmed and trembling.
As for Seungho's demeanor, for now he doesn't need to speak for readers to know that something is off. The beginning of this chapter was clearly like him, taunting, smirking, and so on. But as soon he wanted an intimacy with Nakyum, something was definitely off. If you see his previous sex scenes, you will notice that this one has more skinship than the others.
Okay I've been rereading Chapter 21 and the scene on the last panel is making me think. Na-Kyum whispered "I like you..." in the previous one, and this was obviously meant for Inhun; but in the next scene, we are shown Seungho's face, with the dialogue bubble "I like you..." yet again. So I'm wondering if Na-Kyum repeated himself, or if it's Seungho's declaration to the painter that he is feeling affection towards him - thoroughly taking advantage of the fact that Na-Kyum is drunk and probably won't remember that Seungho ever said it.
What do you guys think? (⌒▽⌒) Am I just bat-shit crazy? Lol