Aight hear me out,nishijima said that he has a younger brother,in that case,twins are two identical person with one older than the other and when we saw his head,I couldn't find the mole under his right eye,what I'm thinking is how did pretty boy know that the mc and the other girls were last one to arrive?my guess is that pretty boy and nishijima are collaborating,theres a chance that the murderer can get much more money if they succeed so they have high likelihood of collaborating with someone and that is pretty boy so theyll give each other information,I think nishijima convinced his younger brother to come to be evidence that nishijima is "dead" hence the murderer is nishijima
(but before he died,didn't we see the killer behind him?)I'm sure that costume can be worn by anybody and that would be pretty boy at that time.when the light turn off I'm sure pretty boy had already killed and brought nishijimas younger brothers body to nishijima then they dissect it and get rid of the evidence leaving behind the head only and they would prepare it where they can drop it on the floor anytime,in this chapter we see pretty boy acting as the leader telling people where to go and im sure that with pretty boy as an assistant ,killing them is more easy for nishijima
so im betting nishijima is killer
