I think it's possible that BK wanted to commit a mass murder, encountered one or more of the victims and began to stalk her/them. For all we know, he may have stalked other people, either other groups of housemates or a family. Unless LE finds writings that explain his motives, all we have to go on is that he did the crime by home invasion and slaughtered 4 people. Had he wanted to kill only one of them, he probably would have stalked until he found an opportunity to get her alone. It could simply have been that they were young, good looking, popular and alive. It could simply have been that he wanted to know what killing felt like and in particular what it means to kill something beautiful, desirable. Why do big game hunters want to kill elephants or leopards or lions? Because it makes them feel powerful.
That's not to say that there wasn't personal interaction of some sort, e.g., taking an order in a restaurant or chatting while in line at a coffee shop. But given no evidence of actual physical or verbal contact, whatever was going on was going on in BK's mind.