American Psycho is set in the late 1980s and told from the point of view of Patrick Bateman (Christian Bale, who also serves as a narrator). Bateman is a 27 year old successful and wealthy investment banker living in Manhattan. He spends most of his time engaged with other shallow people in the Wall Street elite or meeting his fiancee's equally shallow friends. He has a standard morning routine of doing stomach crunches, using specific scrubs in the shower, applying an herb-mint facial mask, as well as using an aftershave with little or no alcohol because "alcohol dries your face out and makes you look older". Bateman is very concerned with his physical appearance.
He's also a serial killer.
American Psycho does not simply engage in gratuitous violence. Instead, it uses violence as a way to make a point about yuppie American culture. The term "yuppie" refers to an individual with a high-paying job and an affluent lifestyle. It is typically used to describe rich people who are not modest about their financial status.
Almost every character in American Psycho is portrayed as a self centered, snobby person. I refrain from using the word "individual" because the characters hardly qualify as individuals. Bateman's friends barely know each other and they barely know themselves. In fact, one of the most notable parts of the film is how the characters constantly mix each other up. Early on in the film, a co-worker, Paul Allen, mistakes Bateman for another co-worker named Marcus Halberstram. Bateman makes no attempt to correct this and simply goes along with it. He mentions numerous similarities between the two men such as how they do the exact same job, have a penchant for Valentino suits, Oliver Peoples glasses, and even go to the same barber (Bateman insists that he has a better haircut). This leads into one of the most famous scenes in the film, the business card scene. The men all show off their business cards. The business cards are nearly identical to everybody but the yuppies themselves. Everybody is vice president and has the same phone number. Everybody at the firm thinks of themselves as the coolest guy in the room, but in reality they are all interchangeable and ultimately insignificant.
Everybody in the yuppie society is extremely selfish. They are portrayed as superficial people who only care about things that involve them. Bateman actually makes little effort to hide his apparently sadistic nature. He says several threatening and disturbing things out loud in the film, including "I'm into murders and executions mostly" and "I like to dissect girls. Did you know I'm utterly insane?" These comments are always either ignored or misinterpreted. The yuppies are so focused on themselves that they fail to see what's right in front of them. They fail to comprehend that Bateman is a psychopath because it has no direct effect on them. In another scene, Bateman brutally kills a co-worker at his apartment, then drags him outside in plain view in a giant overnight bag. An acquaintance sees Bateman put the very heavy bag into his trunk and his only question is "Where did you get that overnight bag?"
Bateman himself isn't much better. A cold-blooded narcissist, Bateman is obsessed with himself and his appearance. Even while with two prostitutes, he decides to admire himself in the mirror rather than paying them any mind. Bateman frequently ignores his equally superficial future wife, referring to her as his "supposed fiancee". In another scene with his fiancee, he is concerned they won't be able to find a "good table" at a restaurant, in spite of the tables not having any clear differences. Most notably, as mentioned before, insignificant business cards are a big deal to him. He is quickly bothered when his card receives less compliments than the others. When he sees Paul Allen's card, which is apparently the superior card, his face seems to be in a trance, showing a mix of both creepy admiration and envy, despite the card having very little difference to his own.
"Look at the subtle off-white coloring, the tasteful thickness of it. My god, it even has a watermark."
At the end of the film, things switch gears and become increasingly surreal. Bateman attempts to push a stray cat into an ATM machine because the ATM machine flashed a message telling him to. Then he shoots a random bystander and engages in a shootout with the police that ends with their car being inexplicably blown up. Even Bateman's face shows confusion when this happens. He finally calls up his lawyer and confesses to several heinous murders. However, the next day, nobody acts like anything happened. He also discovers that the apartment of Allen, the co-worker he supposedly murdered is completely vacant and up for sale. When Bateman tries to confess to murdering Allen to another co-worker, the co-worker takes it as a joke, claiming that it's not possible as he was having dinner with Allen just 10 days ago.
The ending has led to some people interpreting the killings as only occurring in Bateman's imagination. The idea is that Bateman lives in a world so self-centered that a man will pretend to be a serial killer to get attention from others (as mentioned before, he repeatedly makes grotesque comments) but that still won't grab anybody's interest. The other interpretation, as mentioned earlier, is that the murders are real and the film is commenting on the fact that the yuppie culture is so narcissistic that they can't see what's right in front of their eyes. The novel and the film both intentionally leave this answer ambiguous.
Is American Psycho a biting satire of yuppie culture or a psychological study of a deranged mind? In a lot of ways, it could be seen as both. The film remains popular to this day with many praising the performance of Christian Bale and the direction of Mary Harron. The popularity has remained due to the multi-faceted plot and room for interpretation. Theories continue 14 years after its theatrical release because it's more than just a slasher film. It makes audiences think. It makes them think about the society that they live in and how they live their lives. It's as much a character study as it is a thriller, and that's what people like about it.