One of the puzzling parts of this case is that 4 people were killed, and we don't know if any or all were targeted. Further, we don't know the order of the killings. (LE very likely knows from the blood evidence)
In terms of motive, perhaps it was simply attempted rape gone wrong. It's plausible. Perhaps he knew K and M had the upstairs rooms and planned to sexually assault one of them. A knife is terrifying, good for getting compliance with his goal, and quiet if he has to use it. Surprise, they're both in one bed.
He panics as they scream and silences them. K fights back more and enrages him, so her wounds are worse. As he comes down the stairs, X or E confront him and run back to their room. He chases them and kills them. Or... he comes downstairs frustrated and intent on satisfying his sexual urges and starts checking doors for one of the other young women who live there. Surprise again, two more people in one bed.
The advantage of this theory is it provides a reasonable motive. It's probably hard to imagine that after killing two people he could still be intent on rape, but a psychopath might do just that, and be largely unfazed by the killings. Coming down the stairs he might have thought, "That wasn't what I wanted, but I'm not leaving without getting what I came here for."
This isn't my preferred theory, but it's worth considering.
www.independent.co.uk
