I don’t know who the murderer is but my money is one the duke.
Spoilers for my reasons
1) the asshole crown prince knows the mc is pregnant and he is the dad, that’s also why the crown prince is monitoring what and when the mc eats
(The mc was drugged so that why he doesn’t know who the dad is)
2) the duke seems to have low confidence and desperate for the mc to notice him.
3) the case is putting unnecessary stress on the mc the crown prince does want to be together.
My money is on the duke being the murderer as well. Just my theory (contains spoilers) but I wouldn’t be surprised if the duke was the one the drugged the mc hoping to sleep with him but the crown prince got in the way and slept with the mc instead. The duke becomes so frustrated that he missed his opportunity to sleep with the mc that he starts killing people that look like the mc.
Its so obvious that the murderer is after the mc, now the thing is: was it the spoiled prince or the obsessive duke??