I can see it now. Bum - high off emotion - busts out the closet prematurely. He immediately scolds himself. Oh, fuck why did I do that? But it’s too late. Ji Eun starts screaming. “What the hell?!?!” Sangwoo sighs. There’s really no way to explain this. Sangwoo makes some flippant comment about well, they were building to this point anyway. He restrains Ji Eun to take her down to the basement where he tells Bum to come along.
Sangwoo finishes fucking her. Ji Eun comes down from her orgasm as Sangwoo announces. “You can come out now, Bum.” BUM??? She thinks. Bum shakily comes forward and the restrain scene from above happens.
Sangwoo realizes murdering someone he’s close to socially really isn’t a smart move for a serial killer with a hit list lord knows how long and let’s her go. He uses her to further fuck with Bum mentally and emotionally.