I can recall several mysterious murders that seemed to be the work of serial killers, stalkers or cults, etc. They turned out in the end to be simple cases of burglaries gone out of control.
What if one or more acquaintances of someone in the house or mere attendees of one of the many parties thrown there knew the sliding door was never locked and planned a robbery?
They enter the house, but before they can steal anything, Ethan hears the noise and investigates, rushes to attack them and is quickly killed with the knife. His is the "unconscious" body seen by the roommates next day. The killer/s then rush in the bedroom and struggle with Xana and kill her.
Upstairs, Maddie or Kaylee hears the fight and shouts "What's going on down there?" Pumped full of fear and adrenaline, the killer or killers run upstairs and kill both Maddie and Kaylee. They are not only known to the people in the house, but now they have also comitted two murders and are desperate to not leave witnesses.
They kill the two girls. As they just came to rob the house, they have no inclination for sexual assault of the victims. At this point, fear overcomes them and they flee without stealing anything, leaving the downstairs girls sleeping obliviously until noon Sunday.
Arguments and rebuttals:
"But it was targeted" - A burglary can be targeted.
"But students are poor and have nothing to steal" - Electronics, jewelry, perhaps marijuana... Young people have even been killed for their shoes.
"But burglars don't kill people" - Nonsense. Many robberies have degenerated into mass murder. Especially if the perps are known to the victims and could easily be identified.
"But burglars don't carry combat knives" - Young, nervous, inexperienced macho-type teen or 20s boys might.
"But the victims were all sleeping in bed" - The coroner cannot tell if you died awake or asleep. Also, they have
revised that statement.
"But it was too brutal to be done by a robber" - All we know is they each had several torso and upper body wounds.