But then why did he try to kill yoon and not vasya Moemoekyun
Because Yoon had the audacity to leave? Because Yoon found another lover? Besides Vasya is way too big a fish for him to fry. He'd definitely be swimming with the fishes had he tried to kill Vasya instead of Yoon.
Omg. I think the cousin actually thinks he loves Yoon and is hella jealous of Vasya. Holy shit balls. He's off his rocker.