Only you, can show me this

"Clark turning into the villain makes no sense!!" Like okay did you actively ignore everything in that movie? Clark is not just a victim or some dude downtrodden by life. Mary actively says it to him, he doesnt think anything is his fault! He refuses to take accountability! That is why he is so comfortable in the backrooms. He literally says it! He doesn't want to change.

He can do what he wants to the still lifes. He can do what he wants to his employees. To Mary.

And he thinks he can control himself. That his destructive and harmful behaviour will never actually hurt HIM. And that's also why Pirate Clark attacked him. His own anger and harmful behaviors literally ate him alive.

Not to mention the backrooms themselves is what happens when trauma and memory gets distorted. Clark is literally trapped in a neverending loop of his own memories.

Which is also why he goes back to the memory excerise Mary brought up in the beginning of the movie. Clark is living in his own distorted version of that memory. Tainted and warped by his anger.