People always want to see "motive." What's the motive for some guy to get a high-level hotel room and start shooting people in a concert crowd? What's the motive for somebody getting mad and shooting someone in a road rage incident? What's the motive for someone to sell heroin laced with fentanyl? Sometimes, "motive" is "what I feel like doing." This is just another way, in my view, of projecting typical/normal human behavior onto people who are very atypical. We think because we wouldn't kill without some compelling motive that someone who would commit mass murder is wired like we are and has to have some normal personal reason like romantic rejection. Eric Harris planned a huge bombing massacre at Columbine. His motive? He hated people. He wanted the world to end. That was it. He wanted to KILL PEOPLE.
For what it's worth, I'll believe BK "messaged" one of the girls when the prosecution presents the evidence and the context. If he did, it might just have been research to find out if he could get one of them to talk to him via messaging. But in my opinion, BK killed because he wanted to kill. That's the evidence I see for "motive."