TRUE!!!
Yes, I agree with what you said. I genuinely think the servant wanted to help. He knows that Seungho is obsessed with the painter and that he won't stop any time soon.
He most likely wanted to make it less difficult for Na-kyum and also let Seungho believe that Na-kyum desires him. I think the servant has been around Seungho long enough to realise that he has feelings for him/wants Na-kyum a lot and would be very happy if he believed he managed to make Na-kyum hard.
Of course it's all fake in the end (for now) so when he eventually finds out, there's going to be trouble. I just hope he doesn't execute the nice servant.
you sir are a damn genius
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So from what I've seen, this is my theory.
We know from early chapters that Yoon was suffering from impotence (hard to believe given what a sex-crazed maniac he is), but this can happen. He didn't want to give up sex, but didn't seem turned on by his Orgy Buddies anymore. He needed the Painter's porn.
Likely the reason he's been taking the really expensive herbal infusions was to treat his impotence. (They didn't have viagra back then). No one is going to call it that. Being impotent at Yoon's age would be an embarrassment. An attack on his manhood. So the herbal infusion that helps with "vitality" was a nice way to call medicine to help treat impotence.
The most likely way it treats impotence is that it works as an aphrodisiac of sorts. It gets you very horny.
The old man likely started giving it to NaKyum because he thought it would be easier for the Painter if he was more prone to wanting it. Yoon was going to take him regardless. I'm not sure the old man had bad intentions for feeding it to the Painter.
I don't think Yoon realizes the Painter has been taking his same "vitality herbal infusion". So when the brushstroke instantly gets the Painter hard, he thinks "Wow, this kid really wants it! He keeps saying no, but look at this!"
And the Painter is also confused why he was so turned on by what was happening.
So the old man may have just caused greater misunderstandings between the two.
That's my take.