The actual culprit is usually set up to be someone rather peripherally associated to the main plot for the duration of the narrative, since pulling up a "random" culprit that the audience does not know feels cheap. So you might be right @Seihanda
Man this is such a great read . Hope you guys continue with this series. Also it feels cheap for the so called boyfriend to be the killer if I had to guess I would say we have yet to be introduce to the character of the killer.