The only one who saw them back then was... Kang Seojin, no? So that's why he didn't want Shin Jaehyun to come and perform? Because he... betrayed him?
He did mention at motel that he wanted to destroy both the "lover" and his father, but ... It looks like he fell for Jaehyun in the process without expecting that. Oh what a mess...

He did mention at motel that he wanted to destroy both the "lover" and his father, but ... It looks like he fell for Jaehyun in the process without expecting that. Oh what a mess...
