I might be the only one thinking that he did not stalk specifically either girl, and was rather hunting for a young, attractive, girl(s) in a house with easier access. He might have even thought of his acts being a certain “revenge of fate” or “an avenging sword”, e.g., one gal drank too much, the other could be mean to other girls, etc. We might even hear something like it in court, that he chose the victims for a “reason” - JMO. But IRL these “exculpatory reasons” would be invented by BK post-factum, no matter whom he killed. He’d find information to serve as “the reason” online, because, as we have seen, people posting on the deceased IGs are not always kind. IRL, he was a budding SK, a hunter for women.
From this viewpoint, it is interesting that the article writing about him being bullied at school has an interesting twist - whoever describes the bullying, mentions that schoolgirls were the bullies. It opens the door to explanation of his women-hating. Of course, the journalists write what they are told by BK’s own prior friends. But two questions arise, immediately. 1) the girls in his school were bullies, but not the boys? Odd. And, 2) someone, maybe unwittingly, uses bullying by girls to explain BK’s hatred towards women.
I personally don’t question the bullying. But BK went to kickboxing - this is not something one uses to protect oneself from girls throwing paper on one’s head. This is something one uses to protect from male, often street, bullies.