Willow. I think it would be the most shocking and interesting story wise -- Willow has been in an extreme state of desperation for months. Between cheating on Michael, which is against her moral code, a custody battle, discovering Drew's and Nina's deception, loosing her children, her only norm ii work. Outside home she's a wreck. Drew (and Michael's stupidity) arranged things so that her only shot Willow has at keeping her kids is marrying Drew.
Otherwise, Michael would make the most sense, but I think he's a red herring. I really like the way Rory is playing Michael so close to the vest. I could believe he could shoot to protect his kids -- but I don't think it makes sense for him to be the shooter.
There are others who could be the killer, but none of them would be strong enough contenders to hang this storyline on without it diminishing it.
Karen L.