The more I turn this over in my head, the more I’m convinced that the killer was already inside the house when the four students arrived home at 1:45 AM.
There was at least one vacant bedroom, possibly too, if Kaylee had already moved out. The killer could have easily hidden in a vacant bedroom, knowing that the victims likely would not have a reason to enter that room.
I think he was already hiding in the house because police believe the victims were killed shortly after falling asleep. The last call was at 2:52. The authorities say the murders happened between 3 AM and 5 AM.
If this was a party house, how would the killer know everyone was asleep, unless he was already waiting in the house, and waiting for the house to fall silent?
I think it was somebody who was targeting Kaylee, knew that this was going to be one of her last times on campus, and laid in wait for her to arrive home and fall asleep.