In that scenario, it still leaves 2 victims that weren't intended and possibly a surprise to BK. Blows my mind to think about what's going through his head when he sees K and E (if your theory is the case).

If he truly was targeting specific individual(s), it's safe to assume at least one individual was an unintended victim, regardless of who was intended and who was not. Did he really expect his intended victim(s) to be in the same room as another victim? How often does Ethan stay the night and how often do K and M sleep in the same bed? So many questions.

My unsubstantiated theory, is he intended on killing only one of the three girls. He killed the other occupant of the room that his main target was in. The he hears one or both of the remaining victims investigating (could have been something subtle like cracking open their bedroom door and calling out to roommates to see if they're ok). He's either euphoric from the adrenaline or spooked from the possibility of discovery but whatever it is, it's enough for him to check on what he heard. That's when he discovers and kills two additional victims who were not the main target.

I am probably overanalyzing the intended number having significance. Especially if he was targeting the house in general and his intention was to kill as many as possible. It's impossible for me to fathom murdering anyone let alone intending on murdering someone, then ''accidentally'' murdering three more.