Hey. I am having an argument with a friend about this. I think Lucaon feels different about Kyon since the beginning. He does sleep with him- not anyone else, to start with. But with the whole trauma behind him, he is just not able to treat anybody nicely, let alone somebody who he possibly feels atracted to- it only ads in terms of cruelty and hostility. I might be too willing to see the fairytale, but I think there is chemistry going both directions since the beginning and it is something Lucaon can´t really deal with (a bit of hedgehog personality disorder stuff). He is cruel to Kyon to keep his own smoldering feelings in check.
I remember when i was reading this manhwa as it was being updated, i had always thought that it was weird for Lucaon to start liking Kyon out of no where. However, after rereading it for the first time since it ended, i can see when and where Lucaon started to develop feelings for Kyon. Of course, i hated the beginning because Lucaon raped Kyon, i dont think i'll ever forgive Lucaon for that. But i also think it had to due to his trauma of his entire family dying at the hands of a servant. Since then, Lucaon looks down on servants and see them as trash, so thats why he treated Kyon like that in the beginning. However, as time passes, i guess Lucaon realized Kyon was different from other servants. BUT, i am not trying to justify Lucaon's actions and how he badly treated Kyon in the beginning. These are just my thoughts on Lucaon's character development and i feel like the author did a good job in showing that.