At first I was skeptical that the perpetrator would be another student, but after thinking more about it, I think a disgruntled frat member (specifically Sigma Chi) fits the facts pretty well. The following is just a theory, but I think it makes sense.
One of the possibilities discussed on this thread has been whether or not the perpetrator knew the victims personally and/or was familiar with the house (as opposed to a complete stranger). I tend to agree with those who have been saying that the perpetrator wasn't a complete stranger as it would be extremely risky to break into a house without knowing who could be inside, so I believe the perpetrator was at least familiar with one or more of the victims.
The Sigma Chi house has a direct view of the entire front of the victim's house, with many of the frat house bedrooms having a direct view of the house on King Road (
see this Daily Mail video - screenshot from the video included below). If a member living in the house wanted to monitor the activities in the victim's house, they could easily surveil them from the comfort of his own bedroom––no need to sneak around in the bushes behind the house and risk getting caught. In the dark, it would be extremely easy to see when the victims went to sleep based on when the lights in each room were switched off.
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We know that Ethan (a member of Sigma Chi) and Xana were at a party at the Sigma Chi house on the night of the murders (
ABC News). If there was a fight or some sort of altercation at the party that night, that could be a possible motive. Additionally, compared to Kaylee and Maddie, we know very little about Ethan and Xana's activities that night beyond their attendance at the Sigma Chi party.
Numerous people Kaylee and Maddie interacted with that night (the guy in the hoodie at the food truck, the person they were calling late at night, and the 'private party' driver who took them home) have been ruled out by LE as suspects, but no one Ethan and Xana interacted with that night have ever been publicly identified or cleared (
City of Moscow Police Department press release).
Due to the close proximity of the Sigma Chi house to the victim's home, it would be very easy to walk to the house, commit the murders, and then walk back to the frat house without being detected (no vehicle needed). Additionally, because the frat house is effectively next door to the victims' home, the perpetrator would likely be aware of the habits of neighbors (i.e., whether they frequently have parties) as well as which houses in the neighborhood have security cameras. It also seems that the Sigma Chi house has exterior stairwells (see the photo above), so the perpetrator may have been able to return to his room without coming into contact with fellow members/visitors.