The film is, after all, about the sublimation of human nature into financial capitalism, the commodification of our humanity. Patrick Bateman stands outside of this by some deficiency that has caused him to realize the contradiction between wanting to simultaneously fit in and be recognized for unique qualities. By day he is a misogynist who sits around in clubs and smokes cigars with the good-ol' boys, does no work in his office, harasses his secretary, and by night he acts out the logical deduction to his station in life: he cuts up women.
At first he seems to sincerely enjoy shoving his lifestyle's contradictions in the faces of others. At a dinner with vapid and dead-eyed companions he impresses upon them the importance of both civil rights and equality for women and a return to traditional moral values. Bateman almost seems to be bragging to the audience as he lovingly recounts his daily personal care routine while reminding us all the while he is a lunatic.
He books dinner reservations while watching pornography. He charms a woman into helping him with a blood-stained sheet at the cleaners. Yet after a spell he becomes discouraged and frustrated. As he drags a body bag out of his apartment building, leaving a trail of blood, a co-worker spots him on the street and asks where he got the bloody bag he is loading into a taxi. He openly confesses his murderous pastimes to people over dinner and drinks, only to have himself be (intentionally?) misunderstood time and time again. Murders and executions is heard as mergers and acquisitions. His fiance pretends not to hear him when he emphatically expresses his need to engage in murder and torture as often as possible.
Bateman's distress (and his lust for murder) grows as the subsumation of human qualities becomes more and more apparent over the course of the film. Women express the desire to be mothers, but are left high on xanax or holding pot-belly pigs or are otherwise enticed by money. Men are unable to assert masculinity by work they have done or accomplishments of merit - they are left comparing business cards and bragging over who can make more exclusive reservations at upscale restaurants. Drug use functions as it should in America: they lower inhibitions or else numb the pain of being alive.
As the film draws to a close, the action comes to a head as Bateman goes on a killing spree after unsuccessfully trying to feed a kitten into an ATM machine (this is deep). He blows up two cop cars and hides in one of his victims' apartment while a police helocoptor hovers outside. It is then that he leaves the cathartic confession on his lawyer's answering machine. He figures: the jig is up!
Yet the next day we find that either Patrick Bateman is insane (very clearly he is, but...) or the world is complicit in his destruction. He returns to the apartment where he kept the bodies and finds everything painted over and up for sale. His buddies at the club seem placid and his lawyer, as mentioned above, is embarrassed at his outburst confession. The only indication we have that it is not an illusion is his secretary going through his day planner and finding page after page of dismembered female body parts. Patrick Bateman the psychopath, it seems, is not at odds with Patrick Bateman the financial capitalist. Neither is his behavior or the subsumation of it into his seemingly all that unusual.
This is reinforced by the ending scene, where Bateman and his pals watch Reagan lying on TV about Iran-Contra. Just as it was so obvious to everyone that Reagan was no "harmless old codger" so too has Bateman's confession meant nothing to the audience or to the world so affected by his rampages. "Inside doesn't matter," he tells the audience, and so it hasn't, doesn't, and won't in that world. His structural purpose is all that matters. No one remembers him or who he is, but his part to play is so important that someone else will always hide the bodies for him.