It's not even about where hyungnim had kissed him, this is a prince we're talking about! His body is "royal", and for a mere scholar to place his lips anywhere near that body just means he crossed a line: he went against everything he knew–the teachings, the rules, the traditions–and let his affections for him win. And he knew it was wrong because he could only do it when chang geon is asleep, and the fact that he had drugged him too?! Double whammy. !! It's so conflicting, because by the act of kissing he admitted to himself that he had felt SOMETHING (whatever it was he felt is another matter entirely) and by doing it unbeknownst to chang geon is another admission in itself–that he couldn't let chang geon know!
I'm so conflicted. I wouldn't call it romantic love at this point, only a sense of ownership and possessiveness over someone that's impossible to be his. Which, at this point is not yet mirrored by Chang geon, because the kid lets go of hyungnim and even roots for his happiness, and YET by setting chang geon free albeit cowardly he unlocks something in chang geon which causes the boy to thirst for power. And I'm not at all victim-blaming, but this comes back to him later down the road and we all know how that ends. The irony of it all.
(Also, wtf was he running away from—why was there a need to end it this way, why even drug chang geon at all (((IS IT DESIRE !!!!))))