My current theory is that, after returning home, the four victims were together for a time in the common area when the Killer stopped by. They let him in thtough the sliders. He was well inown to them…a boyfriend, ex-boyfriend, frat brother, co-worker. A nasty argument ensued…loud enough for the survivors to be aware, but not clear enough to discern the problem. This makes them important witnesses,
My guess is that, although the killer’s ‘issue’ is with one of the victims… the four victims were all on the same ‘side’ of this argument…and the Killer therefore became enraged at all four of them. They tell him to leave and he does, through the sliders again.
Now they go to their respective rooms. I do think the girls might have been sharing a room, because Kaylee might have already moved her stuff out and was just up for the weekend. After this confrontation, the girls try to call J the Ex.…either to tell him about the fight, get his advice, or to smooth things over if he was the one they were arguing with. My guess is he has a rock solid alibi from the affectionate way the sister refers to him. Anyway…as we know, the girls had no luck with those calls.
The Killer…infuriated goes home, gets the knife, and lets himself back in through the sliders. Due to the excitement of the confrontation, no one remembers to lock it again. He kills the 4 who he is still raging at…has no interest in the un-involved roommates downstairs. This coincides with the “targeted” remark by LE and why they felt initially that no one else was at risk. In other words, this was not a stranger thrill killing.
Those surviving roommates sleep in. A friend comes by…through their lower door. One of them goes upstairs, sees something, and they all panic and the visitor uses the first phone they grab. They are outside the house when LE comes.