BK would make every effort to not leave traces of stalking, etc.--criminology background and all. He went directly to the 3rd floor, Maddie's room. Others have said that the interior of the house is not intuitive, the way the stairs are. Maybe he found plans of the house layout, or maybe had been inside before, undetected. Based on his OCD I would imagine he planned which person and which house weeks or even months before. Sounds like he is a germaphobe too, sort of surprised he would feel comfortable committing a crime like this at all.
We do not know what he did in the house, he could have wandered through kitchen into living room, he could have gone in all the upstairs rooms before he found Maddie and Kayleigh, the only person who knows what he did that night is him. The two surviving roommates were drunk, terrified and have given many different interpretation of what they heard and what they remember from that night,
I for one after reading the documents am utterly amazed that BK was not seen by many people that night, many students would have been wandering home from parties, the door dash driver and he were feet from each other at some points going by the times,
The housemates had barely gone to bed if we piece together how they were up and down the stairs in and out of each others rooms, taking the dog outside,
It amazes me that he got on and out with only Dylan seeing him one time, and being caught on cameras,
And he for sure is no criminal mastermind, he had no idea who would be in that house that night, he wasn't there when the students arrived home, for whatever reasons he has and IMO there can be many reasons why someone chooses to murder he chose that house, anybody with an ounce of common sense would not have chosen that house, but he did. and unless he talks and says he chose a victim/s to target I will firmly believe he chose the house, he may have known who lived there and they being female were his choice of victim (who knows if he even knew of Ethan) but that night. he wouldn't have known who was present in the home, and in what room they would be in,
I don't think he will ever reveal anything to the public, but he will enjoy speaking to behavioural scientists, profilers, academics, people who he thinks are his equal, but what he tells them will not be revealed to the public