Idt its about the concubine fucking another guy. It was the "I love you" and the audacity to be chummy with him that pissed him off. Him killing off the other guy was a warning to not test him and not really out of jealousy.
I think also even if he's not interested in the concubines, he wants them to not engage in infidelity because in politics, it's basically questioning his authority as the owner of the harem. If he can't even control his harem, how can he control his kingdom. So it's like "I kept you alive with your high status as a concubine even if I don't want to. The least you could do is be obedient and loyal to the crown"
i saw it as the king doesn’t like how the concubine believes he can have power/control over him because he “loves him” meanwhile doing unfaithful things and talking down to his lover in the previous chapter. in comparison, the king’s lust is literally killing him yet he quells the desperation as much as he can. he’s not only waiting for the queen to want him back, but he feels his love and pleasure is reserved only for one special person
then it seems to be an issue because the king was fine with the concubine living quietly but he keeps doing things that interrupt or disturb his time with the queen/clearly making the queen disturbed. his spy most likely has been informing him of the queens reactions towards things including the concubines provocation. but we also have to remember the mental state he was in, believing the queen also thought he was disgusting and untouchable so he was already triggered and needed an outlet for his violence
he truly does not care about the concubine, and whether or not he’s unfaithful. the killing was a warning to stop being so bold and live without stirring up problems
the concubine probably thought his old tricks would work like it did on the last king, like he could weasel his way onto the throne using his sexual nature and body- but it completely backfired because shura doesn’t give a fuck and only care when it bothers his lover
Hmm... still confused. Why does he care if a concubine does it with someone else? Is it a status thing? Like you're not supposed to? But he did let it happen once and it was the norm when his father was the ruler so it doesn't seem to be an issue. Shuraka doesn't care about the guy but still doesn't want him to take other men? Why? And also the concibine... what was he trying to do? Make Shuraka jealous? Seems stupid to believe that infidelity would work in your favor.