BTW Yooyoung and Yooyeong here refers to the black haired guy who abused our cutie patootie Kwon and the white haired guy is Yooyeon (I know both arent same name and didnt look at the name itself, i'm too engrossed in this theory). Another thing is that Yooyeon may have had murdered the Yooyeong of past but the Kwon of past greived of his passing as he could have been brainwashed by Yooyoung despite the abuse. having seen that scene, Yooyeon decided to pretend as Shin Yooyoung when he took over the body, but his behavior, especially unable to remember one self and his friends gave him away to Kwon who's upset of Yooyoung's death being caused by his curse but also feeling cheated that Yooyoung he knows is not the one he's now living with, but he slowly embraced the Yooyeon-Yooyeong as he's the one who brought warmth and support to him when he's lonely just like how Kwon of past brought warmth and support to past yooyeon when he was lonely
TL;DR of the thoery, the curse is part of Yooyeon's plan to protect Kwon from Yooyeong when he's reborn and Yooyeon's decision to take over Shin Yooyeong is to prevent the past Yooyeong to awaken and harm Choi Kwon and to help break the curse
I'm very much excited as to how this story progresses, for people who are desperate to get a full picture, from reading comments and the manhwa (upto 34 as of writing), here it is
In the nation of Joseon, there once was a boy called Kwon who was took in by a rich lord's household to be the young lord's bodyguard, the family had a secret that they wished to hide, or more accurately, a vermin who they wished to hide from the world lest they risk attracting curses and cruel chatter, yet that vermin isn't anything but repulsive, a well mannered, white haired, radiant looking child called Yooyeon who is allowed only on the nights, he had a brother called Yooyeong (could be..maybe??, defenitely) Kwon and Yooyeon met every night to play and chatter as the Yooyeon who's someone better suited to be around people was banished into dark annex of the house, grew lonely and found solace in the nonchalant Kwon's company, but alas Kwon had to abandon him and he soon ended up becoming what he meant to be, Yooyoung's guard and in a cruel twist, became his "wife" enduring cruelty everyday, many moon passed and in one first night of snow, Kwon and Yooyeon reunited, Kwon who regretted his action of driving Yooyeon and Yooyeon unable to be seperated from Kwon, embraced each other with nostalgia and bitterness
Somehow, Yooyoung dies and Kwon cries and the memory of the past haunts the present Kwon which ties him down like a curse, misfortune befalling on people who he comes close to, unable to bear it, Kwon chose to withdraw and be recluse until one day, his only friend/acquaintance Shin Yooyoung dies in a accident when he's with Kwon, once again scarring it, but Yooyoung survives (not exactly) but it turns out, it's not Yooyoung, it's Yooyeon
Perhaps the curse of Yooyeon fell on Kwon who had to endure the curse of people suffering and becoming lonely, maybe but another curse is also mentioned which could be that Yooyoung and Kwon were bound together and that may have forced something of a connection between Yooyoung and Kwon but the Yoyoung is now Yoyeon who now wants to attain Kwon and take revenge on Yooyoung for forcing seperation maybe and finally providing the peace Kwon deserves.
Now here's my theory/take
I think the curse Kwon is suffering from must have been the same curse Yooyeon in past suffered and the seperation must have been forced by Yooyeong as he must have felt jealous of Kwon and Yooyeon and decided to forcefully attain him to fully conquer Kwon.
Having watching this atrocity, Yooyeon must have placed his own curse on Kwon to prevent people from taking him and conquering him since people cant take advantage if the misfortune befalls on them first, but it have caused new issue. I dont know whether this story delves more into supernatural stuff apart from past life and spirit transmigration or host takeover, but for sake of this theory, let's assume that Yooyeong must have had done some sort of black magic or something to bound Kwon to himself and that's how Shin Yooyoung was able to find Choi Kwon in present, despite Kwon's attempt to ice him out, Yooyeong approached him, while it's unclear whether present Yooyeong is awakened, but we can safely assume that that could be case and if the accident didnt happen, Yooyoung and Kwon may have went ot a secluded place for their summer outing and who knows maybe he could have taken advantage of his situation
The curse is somewhat of a blessing in a disguise, however only for this moment, and Yooyeon decided to take over the Yooyoung's body, Yooyeon's decision to lie to Kwon is that Kwon may have not known what's happening and assumed that past Kwon is brainwashed by past Yooyeong which is now tormenting present Kwon, so he decided to play the role of Shin Yooyoung to get close to Kwon and protect him and lift the curse placed on him, but Kwon learnt the truth that Shin Yooyoung isnt Shin Yooyoung but Yooyeon, this must have casued panic in Yooyeon who's now worried that Kwon would run away and he'll get in danger, that's why he's so desperate to hold onto Kwon, but from the looks of it, Kwon is okay with it, since he learned that Shin Yooyoung's death isnt caused by him (spoiler, it is...perhaps it could be), he's relieved and is now starting to accept the new Shn Yooyoung who is Yooyeon. I'm sure that this story would be dealing with Kwon and Yooyeon dealing with their differences and working out the identity and how Kwon and Yooyoung would embrace each other while navigating the curse and the spirit of Yooyoung