PF is a narcissist. There are many varieties. Yes, he needed to hurt her. Likely because she began to see through his lies and realized the relationship was going nowhere fast. He was dragging his feet on making a solid commitment to KB. She was maintaining the status quo until she could figure things out. Narcs lie, a lot. When the relationship is "fresh" they "love bomb" you. Hard to resist. You put them on a pedestal which is their "supply", in essence YOUR love is their supply. Your love is their life blood. They are vampires. Truly. They will suck the life right out of you even if they don't kill you. Kelsey "woke up" and realized her knight in shining armour was a fraud. This process takes a while, and when PF began to feel she was going off him, or perhaps even told him she'd had enough, she wanted more from the relationship, that's when it all went south. A narc needs supply and she wasn't interested in enabling him like his momma. She's a self made woman. A pilot with her commercial licence, no less. No small feat. When a narc feels rejected that's when things get really ugly. I think PF is the brooding type, more than the fly of the handle narc, but both can be equally as dangerous. He needed someone to put him back on a pedestal. Maybe he found his next victim at a rodeo in Idaho, stud that he thinks he is. Starts love bombing her and there he is, back on someone's pedestal getting his "supply", full of hate and resentment for the one who "found him out." And then there's control and that's where baby K comes in and the motive to take KB out of the picture.
Like you, my heart aches for little K because no matter how wonderful her life will be with her her mother's family, she has to grow up knowing her father did this heinous act to someone she needed.