Theory 1:the uke killed his parents by making a deal with the devil in the mirror. He sold his soul to the devil 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 that make ur wishes come true. It 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: one of the two (seme or uke) have a twin and that twin died. Maybe when 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
Black Mirror