I've only posted here once or twice so bear with me as someone who's read a lot of Agatha Christie recently, I'm personally thinking a lot more about the psychology of the case, not necessarily who did it.
To me, when I look at it, the single most mind-boggling thing is that there were ZERO 911 calls from the house. If you think about it, for one murderer to take on 4 people, even asleep, is a huge task considering that all it takes is for 2 people to call the police (one to distract the murderer, one to make the call) or even 1 if the murderer doesn't get to the victim fast enough. Given that, IMO, the murderer HAD to know the layout of the house. I don't just mean Zillow, I mean the killer had to know the layout of every room, how the furniture was laid out. Think about it. This thing happened in the dead of night and if the murderer turns on lights, that will likely alert people way before the murderer wants to. And even then, all it takes is a stray backpack full of college books to send the murderer flying and possibly wake someone up. And, here's the important part, this is a party house, yes, but also, I would imagine, a house where girls don't just invite random people into their bedrooms, especially on the 3rd floor. So given the picture of complete darkness as the murderer is going through the house, the murderer has to know where the beds are located in the rooms, where everything is located as well as possible. That's not a random stranger, that spells targeting, which is what LE was reporting at the start.
Now, there are reports of defensive wounds but still no 911 call, which means that the people fighting back were only doing so one at a time. Otherwise, a second person definitely would have made a 911 call even if all they can do is leave the phone on the line, that would have brought the cops in minutes.
For lack of a better word, this feels like someone who has practice; I won't be surprised to find this person might have military experience or background or training of some kind and quite possibly also might have used night vision goggles. The dog, assuming it was in the house, is also a tip-off. Dogs in general are unpredictable and I would guess the murderer is probably someone the dog knows. Sure, some dogs might go run and hide, but not all dogs will, even all the time. The murderer KNOWS the dog will not alert anyone.