I think the worst was Yongwoo because he manipulated and guilt tripped Taejoo by making him believe that he forced him to have sex when he got drunk when in reality Yongwoo fully consented and even wanted it to happen. Had that not happened maybe Taejoo would've waited more. It was the guilt he felt for Yongwoo that forced him to move on and take responsibility by getting in a relationship with him since it was obvious that that's what Yongwoo wanted. Not only that but after Joyoon returned Yongwoo kept on hurting and mistreating Joyoon by showing off his relationship and telling him to leave Taejoo alone. Knowing that he manipulated Taejoo to be in a relationship with him and leave Joyoon. He was the reason Joyoon was suffering as well as Taejoo.
I think the worst was Yongwoo because he manipulated and guilt tripped Taejoo by making him believe that he forced him to have sex when he got drunk when in reality Yongwoo fully consented and even wanted it to happen. Had that not happened maybe Taejoo would've waited more. It was the guilt he felt for Yongwoo that forced him to move on and take responsibility by getting in a relationship with him since it was obvious that that's what Yongwoo wanted. Not only that but after Joyoon returned Yongwoo kept on hurting and mistreating Joyoon by showing off his relationship and telling him to leave Taejoo alone. Knowing that he manipulated Taejoo to be in a relationship with him and leave Joyoon. He was the reason Joyoon was suffering as well as Taejoo.