That's a sick but REALLY fun twist. I can hear the "There Can Be Only One" jokes already.
Until you said that, I wasn't even THINKING Highlander. I just remembered a few random myths/storylines/mechanics where you collect parts of various supernaturals to gain a degree of their power. Heads and hearts of spellcasters seem to be the most fitting (brains contain knowledge, heads can speak; hearts are seats of emotions, emotions fuel magic).
But I can't help but laugh at the Highlander reference. Thanks for catching me on it! *laughs*
I think I'm going to go with the prophesy angle for the actual killer just because the prophecies let me set up a future case for the PC's. Now if only I could find a random prophecy generator that I could set for extra doom with a side of hallucinogens and tentacles.
A red herring that recently occurred to me is a White Court vampire of clan Malvora, nothing like a serial killer to ramp up fear after all. Might have him working the crime beat for the local paper. Sound like something typical PC's would look into?
Prophecies tend to be cryptic, so be as vague as possible if you don't have a generator. Besides, who needs a generator if you can just confuse everyone with random words that seem to be nonsense.
As for the second part: Depends on the PCs. If you have a group of wizards/whatever who have a vested interest in it (victim was tied to them somehow, whether a friend, minor talent, or even a regular at the bar they frequent), then sure, quite possibly. But if you have a bunch of Red Court Infected trying to keep their heads down, Changelings that would rather stay on the run to stay out of trouble, or a group of minor talents too afraid to leave their homes at night. . .might be a challenge to get them motivated.
Either way, good luck!
--Crion