Snipped by me.
I can think of an explanation. Maybe the knife was already in the car and yes it appears he was stalking the victim's but it doesn't mean he intended to kill them that night or ever. Obviously, when he entered the home it seems his intent was to kill but it doesn't mean it was planned.
As for leaving the sheaf. In his frenzied state he could have easily left it there unintentionally. Unless he's killed before then the act of killing, I imagine, is FAR removed from what he's been studying.
Unfortunately, entering someone's house without their permission is already a crime, and entering the house with even a vague plan of committing a crime is also a crime (burglary). The moment he crossed the threshold of the house with a large knife, he entered the zone of premeditation.
Premeditation includes inflicting wounds on just one person beyond what it takes for self-defense. He has no reasonable way to claim that he was using the knife in self defense. Even if he claimed it, one stab would have been enough (then he should have backed down and fled - as he was now way into illegal territory).
Even if he did not plan to kill anyone at all, once he killed one person, he had the option to not kill any more. Instead, he sent instructions to his body to kill another person, walk somewhere else, and kill two more.
TOTALLY premeditation. People are convicted for strangling murders as premeditated because at any moment during the several minutes it took to render death, they could have stopped.
In this case, I think his frequent trips to the neighborhood, his circling of that particular house (4X?) that night, his parking near it, going inside (illegally) and killing FOUR people can only be premeditated (which does not mean planned, but I think he did some planning - as he TURNED OFF HIS PHONE as he left Pullman).
I believe the PCA also says that he didn't do this at any other time for the period they are mentioning. Just that one time, during which time, he committed 4 murders. This has premeditation all over it.