Yeah, probably definitely the killer is that one art club member weirdo. Maybe he tried to investigate him thinking he was a protag in a manga and forgot that criminals are dangerous and you should go to the police first. Regardless I’m pretty sure the “monster” isn’t the one killing. Maybe just profiting from it or trying to mourn the deaths in her own way? I really don’t know; wish she could talk to explain the plot and save us 10 more chapters of mis communication mysteries