Sorry, i wanted to like
I strongly believe your 2nd Theory.
The parents of Jung Yeon (seme) forced him to become some sort of sacrifice to the mirror (demon?) or be apart of a ritual. Tae Joon (uke) tries to interfere but is also locked in the room with Jung (extra food for demon?). Something happens in the room where they learn the 'truth' and ultimately both end up with some sort of power and they kill Jung's parents (as revenge?).
I also have a theory that Jung Yeon's parents may also have had this power, bc they often visited the Black room, which was why the room smelled of smoke. Therefore they where either trying to encourage Jung Yeon to follow in their footsteps, to be like them and get this power (which is why they forced him to watch horror movies), or use him as a sacrifice/fuel to get the power for themselves.
more!!!Theory 1:the uke killed his parents by making a deal with the devil in the mirror. He sold his soul and left it to b trapped in the mirror hence the seme telling him"u already saved me".
Theory 2: there was some demon or something in that mirror in the black room that made the parents and family rich and successful (the whole family is successful even the seme now) in exchange for demons feeding on the fear and having their way with the young seme. And that's why he was so scared.
And when the young uke got in the room, they feeded on his fears instead. Which is why he is so traumatized. And since he wasnt part of the family he could have his own wish which was having the parents killed.
Theory 3: on of the two seme or uke have a twin and that twin died. Maybe thewhen the seme was being naughty he was put in the room and his brother pulled out till that second twin misbehaved??? Idk. But i loooove it