I haven't been commenting much, but I've been reading along. So thanks everyone for all the links and great information.
I've read through the PCA several times and have formed a bit of a different idea of what happened, including motive. Many people have commented how odd it is that a seemingly regular guy would commit a mass murder like this as his first crime. I agree. Not because I think he is a serial killer of some kind, but because it's possible his intent was not murder at all.
I'm coming to believe the intent was forcible rape and the intended target was M. I believe he knew where M's room was from prior surveillance. I believe he removed the knife from the sheath, which was looped on his belt and entered through the slider in the kitchen and went directly to M's room, finding her asleep. I believe he unbuckled his belt in preparation for a sexual attack at which time three things happened - (1) M awoke and fought back, (2) the sheath slipped off his undone belt, and (3) K heard the commotion and entered the room. At that point, he killed both of them.
I think his intent was to quickly exit the house the way he came, out the kitchen slider. Coming down the stairs he heard X moving around and saying "someone is here". Now he has a problem. Does he leave or does he dispatch a witness who may call 911 in the next few minutes? I believe he decided to kill X, not knowing E was there. I think he saw X as he rounded the corner and attacked her near the doorway or just inside the doorway of her room. Then he saw E and turned his attention to him as the bigger threat. I think X was still alive, injured, and crying. When he finishes with E he turns back to X telling her he will help her, but he finishes killing her instead.
Then he walks back down the hall, passing DM's room, and exits the way he came in through the kitchen slider. He showed no interest in DM's room or the two bedrooms on the first floor. I believe his focus was entirely on M's room on the third floor, he chose the most efficient route to get there, and did not expect K to be on the third floor at all. I think he thought he could rape (not kill) M on the third floor and then slip away.
I think that's why this crime seems so sloppy. He planned to commit a rape, in a college town, which is a fairly common occurrence and does not attract national attention and 50 FBI agents. He wasn't planning a mass murder; he was planning a rape and at most a local investigation by a local detective or two.
This is all my own opinion and my general sense of the case at this time.