Theory: Deals with the devil

MulhollandDr.-Theories

deals with the devil

This movie is about making deals with the devil. And when I say devil, I simply mean some form of supernatural evil. Demon, devil, pick your evil. What are the nature of these deals? Well, that varies, but the two examples that we see the most of is Rita/Camilla and Betty/Diane, and with them, they got new lives for their bargain. There might be plot holes in both given scenarios, alright, but they present possible interpretation nonetheless, since Lynch wasn't quite specific in explaining his own vision.

Scenario 1

Core to this theory lies with declining that the first part in MD is to be taken as a dream. It rather portrays alternate reality: Diane had Camilla killed and upon realization, heartbroken, she meets a man with a black book. In exchange for a purse full of money history will be redone to keep Diane and Rita in love - just for once.
An elderly couple arrive at Diane's home, slipping through the cracks in her door, they terrorize Diane, causing her to commit suicide, and commence her rebirth. They fresh-faced actress Betty exactly at the time Camilla is being killed on Mulholland Dr. But this time the devil makes the hit goes awry and Camilla loose her memory so as to allow her to reunite with Diane/Betty. They eventually fall in love, as planned, but begin to discover the demons Diane has signed a pact with. The love story culminates at Club Silencio where the devil shows up to claim his part of the contract - Diane's soul. Hence the Betty and Rita world collapses and Diane's soul descents to hell.

Scenario 2

Rita starts the movie in trouble. At that point she's already made her deal with the dark side, as evidenced by the fact that she has her blue key. She is saved by the car accident, but also loses her memory of the key and her deal. Eventually she remembers enough to take Betty to Silencio and receive her box. Upon opening it, everything changes.
Now Rita is Camilla, and she has the life that (despite her apparent wealth previously) had eluded her before. Now how is this accomplished? By a fundamental reordering of reality. This is not "Hey kid, I'll give you a million bucks for your soul." kind of deal. This is a complete restructuring of existence. That is why elements are still the same (such as the director's divorce due to his wife's affair) while other elements are very different (such as Betty's shift to Diane).
And the shift to Diane is very important. Because aside from screwing poor Betty, it does something else. It puts her in a position of weakness and need. This in turn leads her to making a similar deal with the devil. When she is at the diner with the "hitman", she isn't asking to have Camilla killed. She's asking to have things changed. Maybe to the way they had been before, maybe to where she has all the wealth and adoration, but in any case, to a different reality where her life doesn't suck and where Rita/Camilla doesn't rub her face in the fact that she rejected her.
Yet in the end, another change doesn't serve the devil's purpose, as the bargains have already all been made. So Betty/Diane is pushed over the edge she was already teetering on by her elderly visitors and takes her own life.

Explanations for various points and evidenc of the devil

Old couple haunting Diane

Demons

Man behind the wall

Another man behind the wall

The devil

Smoke at Diane's bedroom

The old couple are guiding demons sent by the devil. Their eerie, malicious laughing in the limo, after they dropped off Diane in the Betty world, clues us into their evil nature.
The Bum in the back of the restaurant: The evil entity behind it all. It uses others to make the deals and manipulate, but its in control. Note the visual connection that's being made by transitioning from inside the diner (where a deal has been settled between Diane and Joe) to the bum behind the dumpster. He is the one unleashing the old couple, the demons that come for collecting Diane's soul. Also, after Diane killed herself the bum's face is superimposed on Club Silencio curtains. He is the power behind it all.
The man in the diner that had the dream: It could be that the man picked up on the evil thing nearby. Either that, or he had made a deal with an intermediary of the thing and the dreams were foreshadowing of his fate.
The man behind the glass: An agent or an element of the force behind all of this. It is clear that he controls to one extent or another the way terms to deals are met, such as in shutting down the director's movie and freezing his accounts.
Director and picking the "right girl": Two things are clear about the Castigliane brothers at the meeting. They're both very feared by their hosts and they are not normal. In all likelihood, they are not even human, but regardless, they are pressing to get a specific girl the lead in the director's movie. Why? Likely because they are fulfilling a deal with the girl or someone close to her.
It's also clear that by the end of the movie, the director has gained continued success by giving in to the demands of the coffee man, who is now at his dinner parties apparently.
Club Silencio is the final destination for all the people contracting with the devil. The Magician with his goatee and the fork (cane) makes Betty realize that this was just an illusion (ordered herself) and gives his victim a glimpse of hell (convulsion) before the world eventually falls apart. A connection between the magician and the devil figure behind Winkie's can be made via the blue box that shows up in both places.
The otherworldy smoke and blue flashes in Diane's bedroom after she killed herself signify Diane's soul decending to hell. After all, the Magician vanished in a similar cloud of smoke and blue flashes at Silencio. The unrealistic heavy smoke after the car crash on Mulholland Dr. indicates the entrance into the alter reality where Camilla actually survived the hit.
The dead body in the bed is Betty's former self, the near to end scene in which she commits suicide is chronologically between the two time paths. Lynch has simply reversed the two halves of the story, so that we discover the demons and Diane's pact at the same time she does.

Hollywood is Hell!

Hell sign

In the scene when Betty and Rita are outside at 2 in the morning trying to get a taxi, there is a pole/street light that has a sign that says "HELL". That sure makes it clear what Silencio Club is all about. The entire message reads: Hollywood is Hell!

Final thoughts

I personally believe that in Fire Walk With Me, Lost Highway, and Mulholland Drive are nuanced and varied takes on how people deal with supernatural forces. In FWWM, this took the form of possession, while in LH and MD, the issue is making deals. In LH, its primarily bad people making deals with the devil, and in MD, its primarily good people. Interestingly enough, it could be argued that the bad people come out better, but both sets are doomed in the end.
One final twist: Perhaps the entire movie takes place in Hell. This would account to the massive shifts in reality, as the characters are tormented and damned by their selfish desires and the failures that ultimately result from the deals they make. - (Tenebrae13)