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I'm thinking Maddie and Kaylee (from the food truck trip) slept in the 2-bedrooms located on the 1st floor. Those 2-rooms are on the backside of the 1st floor. IMHO Ethan and his girlfriend were on the 2nd floor residing. The other two females slept on the 3rd floor. <modsnip>
Also, to make note on the severity of being stabbed, it only takes one deep plunge into a person to immobilize them. Additionally, if the initial stabbing enters the chest, instantly the ability to yell or scream is limited if the knife enters a lung and or the depth of the knife going in can shock a person enough to stop them from fighting. However, Xana's father stated that his daughter gave an incredible fight considering what the autopsy report described, bruises and tears from the knife. So, I'm assuming the 1st floor girls were murdered first. Ethan may have heard something and got up to see what was going on, he instantly got attacked at the 2nd floor bedroom doorway and died on the floor near the door, Xana had enough time to attempt to fight back, the battle that he got from her took him out of the original plan of going up to the 3rd floor and taking everyone out. However, he may have felt that the noise from Xana's fight was going to bring the police dept there shortly or maybe a neighbor could have woken up from the noise. Therefore, I don't think he had enough time and was more afraid of getting caught from the current blood bath that he created on the first two floors thus far. IMHO
My assumption, is that the person who committed the crime, has an eyesight view of the back side of the house from a side street or some place that is inconspicuous to hiding this person's creepy behavior, possibly a peeping tom, or just straight perversion. This is not a crime of passion, it's 100% premeditated, carrying around a 6-inch knife is not the average size pocketknife that guys carry around to cut their nails, etc. This crime was planned out and it was a solo job. The assailant knew what they were doing, had an extremely sharp knife, and knew how to use it and were well aware of the vital points in the human body. It only takes one stabbing into a human body to immobilize and silence them. The only person that had a chance, was Xana and she almost succeeded, but her last fight in life possibly saved the other two girls upstairs. That is how the surviving two roommates should be viewing the luck of their survival. My gratitude in life eternally would be going to Xana's family for raising such a strong independent girl that may have redirected this sick person's original plans. IMHO