My speculation about this entire tragic ordeal is that both KK and PF are narcissists. KK was so infatuated with PF because he was a challenge to her. PF thought of her as someone who could be easily manipulated. In a way, I think that he underestimated her.
I believe that PF thought that KK was so enthralled with him that she would do anything for him, to include murder. He wanted her to murder KB and possibly take the rap for it if caught.
KK played along with his "game" three times, each time having an excuse to back out. I'm not sure that she ever thought that he would really go through with actually murdering KB if he had to do it himself.
I actually think that this is the reason that he didn't dispose of KB until KK arrived. He still wanted to implicate her in the murder, even if it were after the fact.
We know that KK is not playing with a full deck due to her reaction when PF calls her to clean up the murder scene. Any woman in her right mind would call the police and explain the phone call and the previous requests.
I personally would be scared to death to drive out of state to help a murderer clean up a crime scene, not only because of the gruesomeness but also because I would think that I was being lured to town to be murdered myself.
I guess, by involving KK as an accessory after the fact, PF thought she would never admit to anything. She had too much to lose--her career, her family, basically, her life as she once knew it.
BTW - I think that KK outsmarted PF again when he tried to get her to take KB's body with her and dispose of it. Once again, IMO, he was trying to set her up as the murderer and to keep LE off his trail.
IMO, she will spin this entire sordid story. Her friends and family will buy her version, forgive her, and she will move on without missing a beat in a few years.
She could have been a hero and saved a mother's life. Instead, she turned out to be a narcissistic zero, who, in the end, one-upped her narcissistic challenge.
When push comes to shove, it's every man for himself. PF wanted her to take the fall. She's crowing inside that she outplayed him at his own game.
KB and her loved ones are the losers in this entire mess. Very, very sad.