Here's my take.
It was very cold that night. I suspect the killer had on heavy pants, boots, jacket, gloves, head and/or face covering and was not cut during the fracas and therefore no fifth blood trace element will surface. He carried a large military style knife and a tactical flashlight, but he probably doesn't own a handgun. May have bruising in days following or even discharged some type of bodily fluid other than blood such as snot, urine or tears but that would seem like a needle in a haystack type of luck evidence to secure. Bruises are long gone by now. No blood trail leaving the scene is particularly odd. Perhaps the perp even wore some kind of plastic outer wear like rain gear that he removed before vanishing into the night. Seems calculated and planned and neither spontaneous nor the blood lust of a serial killer, who left the downstairs roommates unharmed.
I deduce that Kaylee was the target, and Madison was necessary collateral damage since they were in the same room on third floor. Kaylee's wounds were substantially more traumatic according to the father, which would support some type of personal grudge. Further, the couple on the second floor was likely unintended consequence of a confrontation while fleeing the scene. Kaylee's father shared the coroner's comment that there was a "battle" on second floor/bedroom. It's also been reported that Xana had defensive wounds. So Ethan hears a disturbance, goes out into the living room to investigate and comes face to face with killer descending the stairs who attacks with effective precision and confrontation ends in bedroom where Xana also is killed during the melee. Killer closes both doors after attacks which auto lock with the digital locks. This is a futile, panic response from the perp in an attempt hide the shame of the crime and from casual observation. To me this screams amateur actor. Hence, hours late roommates downstairs observe or sense evidence of foul play and call friends to come to aid before one possibly faints during the ultimate 911 call.
Perp fled on foot, probably through second floor sliding door where he likely entered. He may have been hiding in waiting either in the back yard woods or even inside the home prior to their return. The murders commenced between 3:20 and 3:45. K&M had phone activity until almost 3 am, and in the cop's body cam during the MIP/PI event on the band field only a few minutes before 3:00 am you can see the second floor room light turned off (where X & E were found). So 20 minutes or more to pass out/fall asleep and then he strikes. Early risers are generally 4:30 ish so you'd want to be out of there by 4. He lives nearby or possibly escaped to a car in the vicinity. Maybe it's the white car. He's local, strong and psychopathic, and he left the area immediately or quickly enough the next day to dispose of the evidence. This was his first crime of murder. Further, he knew the dog inside the home well enough to neutralize his presence, which means one or all of them knew the killer.
This case doesn't seem that difficult to solve, yet it's gone cold and I don't expect an arrest any time soon if ever. I do not expect any of the early suspects circulated to be the killer. He's one or two degrees of separation, and possibly not a student. His family or friends are weary of recent behavior but do not suspect that he is the killer, and are keeping quiet. If he's a student then he will not return for next semester. If he's a local he will or has moved without notice, meaning he'll pay rent for a month or two to avoid suspicion and then vanish. He's definitely following the case anonymously on Reddit. He wants to know how close the investigation is to zeroing in on his description/identity, and he's emboldened every day that a theory describing him precisely isn't hatched. He will act out violently again until he is caught: think road rage or ambush assault rather than aggression at work, bar fight or domestic violence. He wants to keep a low profile, maintain his anonymity, but he has now killed and it will manifest in another act of violence.
