Sooooooooo in the past Yooyoung was maybe a Tsundere and had a something with the MC, while the MC was also close with Yooyeon. In the present Yooyoung was a nice guy who really liked the MC and so became a victim of that "course", but the moment he was supposed to die Yooyeon took control of Yooyoungs body. This leaves the question why the MC is suffering from that course. My guess, Yooyeon had special abilities or something, was treated as a "monster" by everyone and the only person he had a bond with was the MC, at some point he might've become kind of obsessed and finding out that the brother he dislikes sleeps and in his eyes mistreats the guy he likes, made him go crazy, killing everyone. But the MC had some kind of feelings for Yooyoung and thought of Yooyeon as a friend, so seeing one of them going crazy because of him, killing the other, he couldn't take it and might've killed himself wishing that no one would've ever liked him. This in combination with Yooyeon not being able to accept his death and the fact that "karma-wise" all the deaths backfired on Yooyeon, but also on the MC lead to the MC being cursed in his current life.
Sooooooooo in the past Yooyoung was maybe a Tsundere and had a something with the MC, while the MC was also close with Yooyeon. In the present Yooyoung was a nice guy who really liked the MC and so became a victim of that "course", but the moment he was supposed to die Yooyeon took control of Yooyoungs body. This leaves the question why the MC is suffering from that course. My guess, Yooyeon had special abilities or something, was treated as a "monster" by everyone and the only person he had a bond with was the MC, at some point he might've become kind of obsessed and finding out that the brother he dislikes sleeps and in his eyes mistreats the guy he likes, made him go crazy, killing everyone. But the MC had some kind of feelings for Yooyoung and thought of Yooyeon as a friend, so seeing one of them going crazy because of him, killing the other, he couldn't take it and might've killed himself wishing that no one would've ever liked him. This in combination with Yooyeon not being able to accept his death and the fact that "karma-wise" all the deaths backfired on Yooyeon, but also on the MC lead to the MC being cursed in his current life.