If neither Kaylee nor Ethan were residents of the house, then they were not the target.
If Xana was targeted, there was no need to go to the 3rd floor.
Logically, Maddy was the target. It was not a spur of the moment event, it was brewing for a long time and the semester was about to end - hence the timing. The perp is known to all of them as a casual acquaintance, a friend of a friend and he has been to the house. He came in through the front door and went straight up the stairs to her room. Kaylee was a surprise and became collateral damage. Exiting through the rear doors, Ethan or Xana saw him walk by their room and they are dead because he thought they surely recognized him.
It's one perp whom drives the white car and left a bucket of forensic evidence behind. It is being processed. This will be solved.
MOO