Why does Holden envision himself in a movie scene shooting Maurice with an automatic weapon? What does this fantasy suggest about Holden?

This particular passage in The Catcher in the Rye comes at the end of Chapter 14, following Holden's interaction with the prostitute, Sunny. Holden arranges with Maurice to have a prostitute sent up to his room; when she arrives, she turns out to be a very young girl, Sunny. Instead of making Holden feel less depressed, or more grown-up, or "sexy" as he puts it, Sunny's presence just makes him feel more depressed.

When Sunny...

This particular passage in The Catcher in the Rye comes at the end of Chapter 14, following Holden's interaction with the prostitute, Sunny. Holden arranges with Maurice to have a prostitute sent up to his room; when she arrives, she turns out to be a very young girl, Sunny. Instead of making Holden feel less depressed, or more grown-up, or "sexy" as he puts it, Sunny's presence just makes him feel more depressed.

When Sunny returns with Maurice, to collect the additional five dollars they believe they are owed, Maurice insults and punches Holden. Instead of the interaction with Sunny alleviating Holden's depression and helping him feel adult, the events of the night leave Holden feeling like a helpless child:

All of a sudden I started to cry. I'd give anything if I hadn't, but I did.

When Maurice and Sunny leave, Holden, in his mind's eye, imagines exacting revenge on Maurice by shooting him, as though in a scene from a movie. This is very telling of Holden's state of mind. Holden's reasons for wanting revenge on Maurice are, on the surface, because Maurice insulted Holden, for striking him; but ultimately, what Holden is most upset about is the way in which his interaction with Maurice violated and removed Holden's sense of control.

Throughout the story, Holden has been suffering from a deep depression and an unsettling sensation of loss of control. School has slipped away from him and he cannot control his spiraling moods. The trip to the city, the hotel stay, and the interaction with Sunny were all attempts by Holden to regain his psychological agency, or sense of control. Maurice violently removes this agency. He deflates Holden's attempt at bravado, and reduces him--literally--to a crying child. In his mind, Holden sees himself exacting the ultimate revenge on Maurice, thus controlling the situation, and making Maurice pay for what Holden sees as his contribution to Holden's state of mind.