It's been obvious since the day they appeared.
Not to everyone. Obviously.
But why thooo
Okay, here's my take. We know it has got something to do with the queen as many of the victims are somehow related to her, but the queen herself isn't the serial killer since why would she dirty her own hand? Like the last chapter, MC mentioned that the queen wouldn't go out to secretly meet a male serial killer because once found out, she would lose her dignity. That all comes down to one thing with all the things we've seen so far; the serial killer isn't a man in the first place. We already know it's not Renius because why would he use poison and rope to kill victims of he can just harm them directly even if he has a phobia of blood. Although things may lead to the Grand Duke being the killer, he isn't because he has no motive to do so until recently because he wanted to marry MC.
All we know is that MC is destined to die when she marries because the night she dies where was she? At the ML's house, who's the other members of the house? ML's sister and stepmother. Straight off the bat, it isn't the sister since the sister has absolutely no military training nor motive. The only character that has been introduced that isn't ML, ML's father who is dead, Grand Duke, John and other male characters unrelated to this is the stepmother who we believe to have seduced the former duke after the duke's wife (ML's mom) has died. Why would a man be cheating on his wife at a military camp during war or a battle if he could do it somewhere more comfortable? What is a woman doing there is she doesn't have any training? Unless, she doesn't have any military training because she was never a soldier in the first place, but the former duke was the one who taught her all the things we've seen so far. How to defend herself against people of larger build, how to tie a steady rope like a soldier, etc. Maybe he never taught her, but she could have seen him do it.
Which comes to one point: The killings might be related to the queen as she does benefit from them because she did collaborate with the serial killer. The way she met the serial killer could be by normal tea parties not secretly as in who would care about some private tea party among women? That's how the serial killer has been unsuspected and has been able to enter and leave the palace. I don't know the motive she might be doing this as it might be related to her status as Dowager Duchess or maybe wanting to gain power... I don't know honestly nor do I know why she does the killings herself.
Renius’s momma is the serial killer.