
He must like to tease the boy, and also surely because he's ennoyed to discover that Nakyum likes In Hun.
Why are you so pissed that's Seungho? Would you want instead In Hun to be with Nakyum in such moment?
We know that the teach would never do all that with Nakyum since he despise so much gay people; if it were him that was kissed by NK at that instant, he would reject NK and says again some nasty things that would deeply hurt the painter.
Seungho, as he is, took his chance to sleep with the painter, it might be not the right thing to do, though at least Nakyum was loved during his first time.
They were fated for each other whatever you can say.

Did Seungho really say all of those things to Nakyum? I cant read Korean but if so... (/TДT)/ ... jeez that'd be really sweet. Who knew our local asshole was capable of being so tender??? ╥﹏╥
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That's was hot, hot but sad for both of them. Sad for NaKyum because it wasn't with the person he loved, and for Seungho because he was so gentle, manipulative and annoyed at the same time, it was quite a sight.
The only time NK said 'Lord' was before they came together. But you can clearly see that SH is showing signs of care. He cares so bad and NK will be his obsession.
During the sex scene he was soothing him, telling him it will be ok, he tried to know how he kept his virginity until now and he was kind of proud he was his first. But he was annoyed that NK was calling him 'teacher'. That could be the first time SH was making love instead of doing sex. Maybe he has done it before, only a dive to his past will tell us.
I said in the previous chapter (and I was quite hated for it) I don't condone what Seungho has done but jumping on NaKyum doesn't make him more or less guilty than the later. Lust and alcohol are never a good mix.