Responses
The hands could be the image of his blood stained hands when they were younger going off of your theory he imagined the hands that killed dudes parent but it’s all actually Yeon Woo he just perceives the abundant touching as multiple hands
Spoiler, I have a theory
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One day Joon, our protagonist, went to see Yeon Woo. He climbed up the tree to his room, as always, but he wasn't there. Then, Joon listened to some weird sound coming from the famous room and he may have rushed to it when he heard the screams of Yeon Woo. I don't know what his parents may have done to him but he sure showed signs of abuse throughout the past chapters. Joon got scared and trying to help his friend he could have pushed one of the parents, who collapsed into one of the mirrors that broke and stabbed them (chapter 15 showed the face of a dead blond man beside mirror shards) and then the other parent tried to choke Joon (hence his desire to choke himself when he feels angry) and Yeon Woo must have stabbed his parent with one of the mirror shards to help his friend (in ch15 he has scars in his hand).
I don't know how to explain the mysterious bloody hands that Yeon Woo can control though.