I think someone who takes a knife designed purely to kill humans into another's home, while masked and dressed in black, at four am, while knowing the occupants are home, has at least pondered the act of murder. Why else take the knife? It wasn't a blade he picked up from the kitchen after being surprised in an act of burglary. It was a knife he brought to the scene and stabbed four people to death with, in their beds or at least their bedrooms, and as far as we know, he took nothing but their lives. The motive was to kill. He wasn't chasing them from room to room while they screamed and fought, he killed them where they were sleeping, almost silently. And if the motive was sexual assault, why no mention of that anywhere, at all, in the charges? I agree, the crime was sexually motivated, but I think for him, as I've said before, it was all about the knife. I think the fantasies he had leading up to the crime revolved around it completely. The stabbing itself was the act of rape, to him. It was a complete act in itself, he didn't need to drop trou to feel satisfied. In fact, he strikes me as the sort who might actually be averse to conventional intercourse, all that physical contact and fluids.
Of course, I am not any kind of psych, so this is all just my opinion.