American Psycho/WMG

WMG for American Psycho. Here be spoilers.

Patrick Bateman, for some reason, has a perception filter on him.
Think about it. He's unnoticed, easily forgotten and even when it would be in one's best interest to pay attention, nobody ever does. This is the exact effect that a perception filter gives you. You might want to focus on what it is on, but you just can't. Your mind will ignore it against it's will. This has been proven with the TARDIS, Torchwood's elevator and with TARDIS Key necklaces. Due to some unnatural reason, Patrick Bateman was born with one that instinctively turns on when it's needed. This is what allows him to confess to crimes, make outrageous amounts of noise and not bother anyone and do things that would normally draw attention.

Patrick Bateman is a Widderslainte.
In Mage, a widderslainte is the reality-warping avatar of a dead Nephandus, reincarnated into a human. Patrick is clearly a widderslainte on the cusp of awakening. He's not an unreliable narrator, instead his crimes are real, and literally defy reality (and law enforcement) because he's unconsciously using magic. The final speech is his epiphany as he awakens...
 * Alternatively he could just be a Marauder. Which is pretty equally terrifying.

Patrick is Sylar.
Sylar was still stark raving mad when he worked for a big corporation, but he got better when he quit his job to fix watches. He still kills people, but he doesn't torture them as often, and doesn't eat the brains any more. At least, not without mustard.

Patrick Bateman is Boo-Rishnikov!
He keeps telling people fantastic stories, but it's all to distract from him being a giant chicken!

Patrick Bateman is Kevin McCallister, all grown up.
Not only does he take scary glee in his potentially fatal traps, but Kevin also explains his morning routine out loud. He also comes from the right class background and certainly has a family messed-up enough to give him issues in the long run. Also, everyone forgets him. A huge theme in American Psycho is that few recognize Patrick (if he is who he thinks he is in the first place).

Patrick Bateman never killed anyone
His murders were delusional revenge fantasies. He's psychotic in the clinical sense; as a result, he cannot distinguish reality from his fantasies.
 * Evidence in support of this from the film - the 'feed me a stray cat' sequence is not explicitly shown to be a fantasy there as it is in the book, implying that the other murders are similarly fictitious when it becomes clear that this sequence never happened. Also, it's implied in the film that Paul Allen is alive and well in London. By this theory, he genuinely did steal a lock of the model's hair, but he did not kill her to get it.
 * Further evidence: in several sequences, Bateman gets out a nail gun and uses it on victims. Nail guns are NOT self powered. Almost all nail guns are fired by compressed air. A very few are electric, but only some small ones, not the huge framing guns that he describes using. To use a nail gun, Bateman would have to have a good-sized commercial air compressor and a great big long hose in his apartment. Not only are these never mentioned, if they were actually present, he would have found a way to use them. New York Yuppie Bateman wouldn't know any of this. Secondly, to make a nail gun fire when it isn't pressed up against something requires fairly elaborate modification to remove the built-in safety features. Patrick Bateman is never presented as having any sort of technical/mechanical knowledge that would make this possible.

In the film version, the more elaborate or dramatic the murder scene, the less likely it is to have happened.
This writes out the 'feed me a stray cat' sequence and the murder of Paul Allen, as observed above, as well as the unlikely chainsaw death of the prostitute, while preserving the stabbing of the homeless man, the stomping of the dog, and his initial abuse of the prostitute.

Only the murders where Patrick actually makes an effort not to get caught happened.
Basically, this leaves Paul Owen's murder and some of the hookers. The more likely Bateman would be to be caught, the less likely it would have happened. For example, the child murder didn't happen (killing a child in broad daylight in the middle of a crowd without getting caught seems genuinely impossible), likewise, the cleaning lady who redid Bateman's apartment never happened.

Patrick Bateman never confessed
Every confession Bateman makes is unspoken except for the confession to his lawyer. Confession is at the back of his mind during other conversations. The audience hears it because Bateman is an Unreliable Narrator. For example, Bateman tells a woman he is into "murders & executions." She replies as if he had said "mergers & acquisitions."

The main character is not Patrick Bateman
This seems to be a theme of a lot of WMGs for American Psycho, but this is more general.

A big theme of American Psycho is that very few recognise him at all, and several confuse him with other people or refer to Patrick Bateman as a different person. For instance, at the end, his lawyer congratulates him on the prank but notes that it was a flawed joke because no one would believe that dorky Patrick Bateman would be capable of murder; he seems to be referring to Patrick Bateman as a different person. Assuming the above two WMGs are correct and the main character is quite insane - literally! - this implies that he's dissociating beyond the point of doing crazy things and truly doesn't know who he is and simply believes himself to be Bateman.
 * Further evidence of this is his introduction. He begins by describing where he lives, and then says his name. Most people would start by saying their names.
 * Or, It's a subtle nod to his obsession with social status. It's more important to him that people know where he lives, then what his name is.
 * It should be noted that false identification is a recurring theme in both the novel and the movie; individuals mistake their acquaintances for others at the drop of a hat. Everyone is so self absorbed that they are literally incapable of remembering the names of people around them; it is exaggerated to a hilarious degree. The reason his lawyer referred to Bateman as some other person was because he mistook Patrick for someone else.
 * It's made worse by the fact that every single banker-yuppie wears identical clothing and haircuts (truth in television, especially in the 80s) and goes to great lengths to look as good as possible, converging on identicalness.
 * Patrick has a split personality. People don't recognize him because most of the time (perhaps all the way to the end) he isn't Patrick Bateman. This alternate personality is slowly taking over the real Patrick; the real Patrick may be aware of this. This is seen when he notes that his confession is worthless because he never committed any of the murders. He, as a person, will soon cease to exist once his alternate personality has totally overshadowed him.

Patrick Bateman is in hell
The book's opening and closing lines, 'Abandone hope all ye who enter here' and 'This is not an exit' reference Dante's Inferno and Jean-Paul Sartre's No Exit. Patrick is actually dead and in hell. Every other character in the book is a demon, and are purposely letting Patrick get away with his crimes, and acting like shallow morons to drive Patrick insane, as part of his punishment. The graffiti and the sign on the door at the end is basically the fabric of hell taunting Bateman by reminding him that there literally is, No Exit.

Patrick Batemen is killed in the Police Chase and is in hell
Related to the above, but a little more in depth: No reference is made in the book to how Patrick survived the police chase and escaped, especially given that several people saw him kill a police officer. It's likely upon cornering him, they simply took a kill shot. From then on in the book, he doesn't murder anyone. All of his horrible actions, from then on, are turned inwards and are on some level, self destructive (drinking his own pee, making a necklace of bones and staying home all day to masturbate while wearing). The cab driver, IE a non-human in Patrick's eyes, is the only one to acknowledge his actions and he does so by robbing him. All of his friends simply ignore his confessions. Trapped in this yuppie lifestyle but unable to access his outlet (IE killing people) is his punishment.

Patrick Bateman is a Waste-Lock.
Waste-Locks are beings who are used to seal all of the negative human energy in the world into glorified trash dimensions. Usually, they're of no threat to anyone but themselves, but occasionally they dissolve into a more violent kind of madness. If Patrick has actually committed the murders, this could explain why he's never caught.

Patrick Bateman is Bruce Wayne
Before becoming the Batman, Wayne wanted to experiment with duality and the ability to hide a secret life as part of his globe-trotting training. He was still very messed up about the whole "parents getting killed right in front of me" thing, so the exercise got dangerously out of hand.
 * Something a bit more likely:

Patrick Bateman is The Joker.
Following such a strict order and living the way Bateman does, it'd be a wonder if anyone didn't go insane from his routine. He eventually went insane and changed his look to match in order to be "different." The Joker thrives on inducing chaos into systems, which is what Bateman seemed to be trying to do throughout the story. His reasons for not liking Batman besides the whole "He's a hero" thing probably also is because "Batman" is one letter off from "Bateman" and reminds him of his once orderly self.

Patrick Bateman is currently working for the Cheiron Group in the World of Darkness
There's a description of an archetypal Cheiron hunter in the Hunter: The Vigil core book. Who happened to have a taste for torture, murder, and Phil Collins. They offered him a job, but it didn't take him long to figure out: this is not an exit.
 * Eh, it's more of a running gag in Hunter than anything else. In fact, Cherion hunters often reference the movie for FPD personnel who become slashers, calling them "Bateman's".

The Events of the Movie sequel is only a fantasy of the protagonist.
Her Codename Was Mary Sue doesn't even cover it.

Patrick Bateman is a relative of Norman Bates.
Bates's relatives changed their name to Bateman to avoid association with the murders.

Patrick Bateman is The Slender Man.
He's got the business suit. He's got the completely unrecognizable face. He murders people. Granted, he doesn't use tendrils, but the warning signs are there.
 * He may be a larval form of Slendy.