Early in the novel, the mood is a reaction to Patrick Bateman's absurd lifestyle. He describes in detail his opulent, sterile apartment, his rigorous and self-flagellating routines, and the investment banking set with whom he drinks and gets dinner. Patrick's shameless vices and luxury create a mood in the reader of either jealously, disdain, or admiration. As a first-person narration, the mood of the novel follows Patrick's own, as well. The narrator's ennui and frustration, surrounded by friends he despises and disdainful to all the world around him, creates a gloomy, tense mood.
Later on, as Patrick describes his various murders, rapes, and dismemberings, the mood becomes disgusting and sometimes terrifying and shocking. Patrick describes his attacks in just as much garish detail as his apartment; these bloody, inhuman scenes are so graphic that their mood is simply horrible and entirely off-putting. Occasionally, though, Ellis depicts the sex and gore as so ridiculously forceful and violent that it might come across as dark comedy to some strong-stomached readers.
Near the end of the novel, Patrick's anxiety and addiction get more serious. Patrick's narration becomes confused as real life mixes with hallucinations and dreams. Patrick also learns near the end that Paul Owen is alive and well in London, casting doubt on the chapter when Patrick described murdering Owen in great detail. It is unclear by this point in the novel how much of what Patrick has described actually happened, or whether Patrick was being truthful to the reader. The mood, likewise, becomes obscure. is Patrick in fact only a man with a troubled mind and an imagination prone to vivid, violent hallucinations? Then the mood might be one of pity for Patrick, a man so overcome by delusions to imagine a whole new terrible life for himself. Or is Patrick really a brutal murderer and rapist who lies openly and insidiously to everyone around him, including the reader? The ambiguities of the end of the novel make it difficult to tell.