KK's story of PF's solicitation, his anger when she didnt kill KB, and PF's ordering KK's to come clean up "her" mess and take care of the evidence, sounds exactly like a bully.
My older brother was a bully. Big brother would think of ways to secretly defy our mean Dad, but he didn't want to be the actual naughty one, so he'd tell us we had to do it. Steal him a candy bar, trespass on a neighbors farm and bring back a rhubarb stalk to prove we'd obeyed, etc. Once while following Dad through a store, big brother whispered "You should steal a pack of cigarettes," then quickly walks in front of Dad where he can be seen and not associated with me if I get caught. When we got to the vehicle, he was very angry that I had not had the nerve to steal them. And he ominously hissed, "You'll have another chance!" I felt very guilty about not doing my "job", so I made myself do it the next place we went, got caught and took a beating when we got home. Big brother's response? Said I was stupid for taking them from a small store I could get caught easier in.
And then I married a bully. Bullys don't want to accept blame for anything, but instead of actually changing for the better, they build a wall of anger towards people so they feel justified in their bad behavior. And those of us who aren't that bright dutifully feel responsible. So one of our children got hurt: "Its not my fault," instead of hugging the child. The toddler chokes on a bite of corn chip: "It's not my fault. I didn't let her have them!" instead of trying to help her. The preschooler spills his milk, the new kitten misses the litter box, a child vomits in the bed: an angry "Hey, you've got a mess to clean up!" And then I started getting a constant angry vibe from him which was, guess what, somehow my fault. And one day it dawned on me, it wasn't my fault. (He was willing to change and we're still together.)
He eventually confessed that the angry vibe was to justify infidelity. So he could feel "It's not my fault, she made me mad so I deserve to do what I want."
So MOO, PF is a bully. It's very believable that he built an anger toward KB, justifying his relationship with KK (KB doesn't deserve me anymore); and also his desire to have the baby (KB is a bad mom so I deserve the baby.)
And I agree with the quoted poster's statement that PF didn't want to be the murderer. I think he's absolutely the one who wanted KB gone though, he just didn't want to be the bad guy. So in typical bully fashion, he somehow made it became KK's job to get rid of KB. Perhaps it was even KK's "fault" that PF got into a relationship with KB, I don't know. KK may have broken up with PF several years ago and gotten married so it's her fault he turned to KB. Then if KK spoke of wanting to get back together with PF and suggested he break up with KB, I can definitely imagine him saying "If you want me, it's your job to get rid her." Or maybe it was, "You're the one who came back into my life and made it complicated, now it's your job to fix it." Possibly KB found out PF was having a relationship with KK, and threatened to leave with the baby. That would "definitely" make it KK's job to kill KB so she couldn't leave.
I think PF's line of thought was, "KB is to blame for my anger, and now she won't give me the baby and leave. It's KK's fault for putting me in such a mess to start with, and and now the stupid woman won't do her job to get me out of the mess. So in the end it's their fault I had to do it. But I'm sure not gonna clean up a mess that's not my fault so KK better get herself down here and finish up.
KK then listed other ways PF reminded her that he'd had to do HER job, and now she REALLY owed him to finish up and get rid of any evidence pointing at him!