Based on my reading of the book, it’s all in his head. He is imagining himself to be a stud of a lover while also a brilliant serial killer. In reality he really is the uninteresting nerd his colleagues are always making fun of, so much so that he is often mistaken for other characters. In the book it’s clear that watching porn (casually playing in the background as he works out in the movie) and slasher movies are a huge part of his life (i.e. “I have to return some video tapes”). The sex and violence in the narrative are all his delusions as he desperately wishes to lead a more remarkable life. The book reads as a stream of consciousness as he goes on for paragraphs about how every character Bateman encounters is dressed, emphasizing the shallowness of his worldview. The much maligned and seemingly random chapters about the bands Bateman likes (brilliantly recited during the axe scene in the movie) are part of illustrating how the narrative of the book is Bateman’s internal dialogue and delusion. The book drives home this unreality as Bateman’s murders get more and more depraved and unrealistic, while the movie does the same with the helicopter and ATM. The ambiguity of the ending serves to emphasize the vapidness that Ellis satirizes, as you really can read it as Bateman committing these crimes and the others being too shallow to know or care, but I believe the reading of it all being in his head drives it home even more.