See, I'[m thinking that the killer was already there as an invited or known guest/friend/acquaintance, and had the time and wherewithal to drop something in all of their drinks or snacks. I know it's speculative, but I can't count how many times in college my roomies and I would have "Just one more!" I don't think they would have been eating after the stop at the food truck, but a last drink at home with everyone wouldn't have been unusual. If he/she didn't wait long enough, that could explain the two with the defensive wounds. They weren't as deeply asleep or incapacitated as he or she hoped. IMO of course.