I really wonder why PF felt KB had to be dead. The only reason I can come up with is that he wanted himself and his mother to have sole custody and felt KB humiliated him. Nothing will make a narcissistic man angrier than humiliating them.
KK was desperately in love with him and he simply took advantage of that. A narcissistic man can never have enough attention or sex.
He lives with mama. I'm betting he has an enmeshed relationship with her. I think she's catered to her not and he could do no wrong.
People like this can be very possessory toward children. He probably felt the same way or similarly toward his child. She's a possession. She's his. And he's not going to allow some woman he doesn't care about to take his kid.
He wanted her all for himself.