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I'm not that anon, but Bret Easton Ellis is a gay man who wrote the novel as a means to express his rage for living a lifestyle similar to the one described in the book. In doing so, it is a projection of homosexual promiscuity and superficial aesthetics onto straight men as well as a means of expressing Ellis's struggles with the influence of his father, who introduced him to the Bateman-esque lifestyle and left a suffocating legacy on him. From a lens removed from homosexuality, it's about the struggle of someone successful who was corrupted by New York but still yearns for liberation through violence against others and pursues this liberation or suffers a breakdown as he's unable to cope with how he can't free himself, depending on your interpretation. The movie refines the work's points on this fairly well.
Consider the card scene and how Paul Allen is treated later. Patrick is capable of killing many, many people. He's capable of killing scores of people brutally and joyfully. In nature, Patrick could leap across the table when Paul Allen shows his card and kill him there, establishing the authority he desperately wants. Instead, he has to play stupid games about cards and music and restaurants and girlfriends. All of the effort he puts into his appearance isn't for some self-satisfying aesthetic desire or to be able to express primal supremacy better. It's so he can continue playing these games in order to accrue material wealth and social status unnaturally. From a homosexual lens, Patrick wants to sleep with Paul and his coworkers and expresses this desire that's incompatible with his lifestyle by sleeping with and killing women. Removed from homosexuality, this misogynist behavior is something else entirely.
>TL;DR
American Psycho is a book by a then closeted gay man to express his frustrations with the paradoxically feminine behaviors required of the generally seen as masculine New York lifestyle idealized in the 1980s, having been written throughout the latter half of the 80s and been released in 1991. The author of Fight Club, also a gay man, did something similar for the decaying white collar atmosphere of the 90s.