On the day this tragic story broke in national news, I went online and searched for images of college girls in Moscow, Idaho and a whole lot of pictures of both MM and KG came up (instagram, facebook, other). IOW, they both had an online presence identifying themselves as students at U of I. Their carefully curated style and "twin" poses were striking and even before we knew their names, I thought they stood out.
I was easily able to find Instagrams for both girls. The Greek restaurant owner denied that BK had ever been there, IIRC, but on what basis, I do not know. It seemed to me that if BK paid cash (as students often do), they would have no record of him nor would he have been particularly memorable unless he asked for something strange. The restaurant appeared on some internet lists of "vegan friendly" restaurants in Moscow, ID. OTOH, if he was stalking MM, he would have gone there for that reason.
We will probably never know, but I am curious as to whether BK had a history of breaking and entering and snooping around, as so many serial killers do. If he intended to SA MM and everything went wrong, he may well have been escalating to sex offender.
If my theory about possible online "interaction" (BK at least perusing MM's and KG's social media) is true, then he may have moved from a form of online stalking to physical stalking. I can see the appeal of that for him. It would indeed be chilling to find out that he had already seen pictures of MM and KG before even moving to Washington. It's possible. The pictures went back in time for a couple of years and included family vacation photos and an awful lot of "personalizing" information, such that a disturbed person like BK could imagine himself in a parasocial relationship with them or with one of them.
It's possible, too, that he knew that KG had moved out (through peeping and stalking). He would therefore have been surprised to see her there (she had a new car).
All speculation. JMO or personal observation.