Police have said that this was "targeted". I understand that to mean that one of the victims was the target. I'm guessing that the reason police say that it was targeted is that one of the victims had more, or different, injuries. Maybe he spent a little more time with one of the victims. That's just a guess.Another poster on a previous thread suggested to me that maybe the killer deliberately left the other victims alive because he was done kiling. I'm not sure if it was the same poster, but another suggestion was that maybe the killer got some sense of power over deciding to kil 4 and spare 2. Paraphrasing but "god complex" was the concept iirc.
If the culprit entered the house via the kitchen sliding door (red crime scene tape on door), and if his target was on the third floor (easy to see into the bedrooms from hill behind house), then perhaps he thought it was best to kill everyone on the second floor before going to the third floor - to reduce chances of being caught. Eliminate witnesses or anyone who could run out of the house before the culprit did. Numbers 5 and 6 are interchangeable.