The idea of two antagonists.
Basically my idea is that there will be two antagonists.
One during the daily life and one during the deadly life.
The daily one will make the stuff more... stressful. Manipulating people to suffice his survival and questioning the actions of the protagonist, like, "You said you will save everyone and that there will be no more killings, but we lost half of the cast already". He will make people trust the protagonist less and less. He will add wood to the fire, like Byakuya does after the second case.
The deadly one will mess up the murders. Like messing with the body, hiding viable clues and information, adding clues that are completely unrelated to the case just to throw the investigation off and such stuff.