I still don't subscribe to the notion that this killer needed to be some sort of seasoned serial murder veteran or ex-military/special forces individual who lost the plot and decided to take out a house full of college aged individuals in a reasonably rural University community.
When you look at who the victims were, the location of their home, supposed time of the murders and a multitude of other factors, I think it's much more likely they knew their killer. Without needing to build a boogie man preying on young helpless victims and came across the perfect opportunity by chance, I can easily imagine it being someone who flew under the radar so to speak.
IMO, I can help shake the idea that the perp is an individual(s) close to, or in the social orbit of one or more of the victims. That is to say he (or they) had been to the house previously. At the very least, he was aware of who lived there in some capacity and knew the general living situation of some or all of the occupants. The perp(s) would likely be an INTJ style personality who harboured a killing fantasy, likely for a long time. Someone somewhat introverted and observant, maybe a bit of an outcast socially or was even generally unliked by the "in" crowds within the college community. Alternatively it's someone who was able to mask those sick thoughts and integrated himself (or themselves) well without alerting anyone to anything that would raise any red flags.
Without being overly morbid, think about how this would go down from a potential perp's perspective. If you fit the profile I've outlined above and you were targeting any or all of the victims, all that's fundamentally needed is the weapon and to know the location of the property and have a very basic plan of exit back to a safe location.
Waiting for the "opportunity" would be an incredible adrenaline rush to this person who has mentally planned this scenario out countless times before, and once finally in motion would be impossible to stop. Waiting inside the house itself, or (IMO) the more likely scenario of posting up behind the property in the tree line with a total view of the back side of the house is how I see this going down. In this theory the perp would likely have followed X/E or K/M home, or was waiting behind the house AFTER the 1am arrival of D/B. This would explain why D/B were left untouched, as the perp would have likely watched X/E and K/M arrive only 10 minutes apart just before 2am, and congregate on the 2nd floor together before heading to bed, lights flicking on and off giving the perp the impression that the "house" consisted of only those 4 on this particular night. From this vantage point you would clearly see where the rooms were, and quite possibly even heard laughter and chatter coming from the group before calling it a night.
As soon as the house fell silent long enough for the perp to jump into action, entry is gained from the sliding door on the 2nd floor. Once inside the perp knew there were 4 people split over the two levels. K/M likely taken out first - noise levels at this time are unknown, and the speed of which the killings took place are unknown as well, but I assume it was enough to potentially stir X/E. Regardless of the order, we don't know if there were screams or if it was done in relative silence.
