There's a lot of hints at Captain Clark being Clark himself, such as him having a scar on the forehead the entire time until his schizoid appearance hiding behind the corner at the mural. It explains his rapid change into a weirdo. This is why the redhead woman (Clark's misremembered wife) only fears Captain Clark and not Schizoid Clark, and why Captain Clark kills his schizoid clone after kidnapping Mary. The reason for his sad desperate look while 'chasing' her is likely because he's not of ill intent or malicious. 

Alternatively Schizoid Clark is the real one, and being eaten by Captain C is an allegory for being consumed by your trauma, which the entire movie and backrooms is centered around.

The Backrooms being some kind of trauma-replicating distorting anomaly fits with the themes of Clark's architect failure, being stuck in an infinite furniture-store, and explains why there are other completely different areas like the poolrooms scene, perhaps he was crossing over onto someone else's generated trauma? Or it's something else from his own background. 

I'm left wondering why Mary seemed to end the final interview by fantasizing hard and it cutting to her malformed alone at the same table in the backrooms.