I'm sure people will stick around, second time's the charm ya? XD 

EDIT -- I have a couple ideas for each scenario, not sure if you have something already worked out for each but here's what I got. :) 

If it was a serial killer, then it'd add the component of multiple killings and a sense of urgency to figure out who s/he is. May I suggest if it's a serial killer that s/he kills the victims with a signature or some sort of pattern. i.e. They find the woman's body shot in the head but mutilated to make a smiley face on her body. Like maybe there are cuts shaped like X's on each breast and then gutted to make a smiley face. Gross but it'd be interesting to say the least. 

If it a monster then we could add a research aspect to the Rp, although we could do thee same with a serial killer. Maybe they have to delve deep into mythology or something along those lines. It'd be crazy if towards the end they ended up face to face with it but since they researched they knew it had some sort of weakness and exploited that. 

If it was one killing but with a significant meaning there could be a bunch of possibilities. We could make it a huge scandal where maybe the woman knew something she wasn't and a large company had to take care of her and now the kids know something is wrong with the system and they somehow try to expose the scandal. 

Just a few ideas, I don't care if they're used or not. They were just on my mind and I felt like writing them down. :P