Why Pirate Clark ate Clark, idea 3
The backrooms are about people not being willing to move on, and they stay in there with half-remembered copies of people they know or have seen and places that they have been. Clark was trying to move on from his bad marriage but is still struggling with his failing business. He cannot make progress and is therefore "stuck." The therapist is also a bit stuck due to things that happened in her childhood, but she is doing a better job of getting unstuck than Clark is. When the therapist finds Clark in the backrooms, he explains that he does not want to leave and he is essentially making a home out of it and now enjoys being stuck, or is at least comfortable with the rut he had made for himself.
Pirate Clark comes in to see Clark talking with therapist and is afraid that Clark is working on getting unstuck or may become unstuck and kills Clark to prevent that.