Honestly the art makes this hard to understand/follow so I'm a bit confused. The police went to a warehouse, for some reason, and a demon or monster showed up, for some reason. Killed everyone, for some reason, then sid shows up and tells the bf to kill him but he shoots himself instead? And now the doctors think the bf killed all his colleagues and then shot himself? So the demon or monster used BULLETS to kill everyone? Did I get that right?
He was delusional, it was a hallucination, since Sid always take Do Gyeong Joon aspect to convince him to have sex. So in the previous chapters you can see how slowly Jinyoung is losing his mid. He thought that he was shooting Sid. The demon that attacked them was there minding his business and eating humans. The police had been because one of the victims called them once they have been attacked by the demon in the warehouse.
I have that exact same fan behind the ducky