First of all, I would like to thank the meme community for making me familiar with this movie. And I'm sure that's how many people who watched this recently got familiar with the film. I would've never known about this underrated masterpiece if it wasn't for the memes.The movie was intended to be rather confusing. The director gives you the responsibility to imagine what actually happened with Patrick Bateman, i.e. the protagonist. The movie does have an unfulfilled ending, but that's what I like about it. I like movies which are left off to be imagined by the watchers about what happened actually.
*SPOILERS*
So I understand why many people call this movie controversial and confusing. Because the movie doesn't seem to have an official description of its own.
What can be deduced from the ending is that Patrick Bateman is an unreliable narrator.
The murders by Bateman were either:-
1) 100% real
2) Totally imagined by Patrick Bateman and hence, an unrealiable narrator
(1) MURDERS WERE REAL:-
This is my first theory. The murders were absolutely real. Patrick Bateman didn't imagine them. He is a psychopath who kills people whom he envies and disgusts. And sometimes simply because he wants to. But the actual reason he does the murders is because he wants people to notice him. It is evident from many scenes in the movie that everyone ignores Patrick Bateman. Such as Paul Allen mistakes Patrick for Marcus, even though they work in the same company. It suggests that Patrick is a less important fellow, and so is Marcus because Patrick is mistaken for him, but that is out of context. Another instance is shortly after that scene itself, Bateman shows his new business cards to his colleagues and they do the same in turn. Bryce prefers Van Pattern's card to Bateman's, even though clearly Bateman's is much better (I think). Another example is, at the end of the movie, Bateman sends Harold, Bateman's lawyer (he is actually more of a lawyer for all the workers of the company Pierce & Pierce) a phone message that he killed all the people. The next day when he meets Harold, he just bursts into laughter and realises that the joke was made by Davis (the name with which he calls Patrick by) and Davis pretended to be Patrick Bateman (This scene is one of the most controversial scenes. What I think is that Patrick once conned Harold into thinking that he was Davis, just like he pretended to be Marcus infront of Paul Allen. But this is an unimportant scene to be honest). After that, Harold says that Davis (actually Patrick) made a fatal mistake in the joke, he pretended to be Bateman (Harold probably thinks someone else is Bateman) who is a "dork" as per him and could never murder anyone. Harold mistaking Patrick for someone else and saying that Bateman is a dork shows Patrick being ignored. Everyone in the company seems to know Patrick only by his name and they all know he is a dork. Only a few colleagues seem to know him closely and by his face (Bryce, Luis, etc.).
(2)ALL MURDERS WERE HIS IMAGINATION:-
Patrick is ignored by everybody in his company, and is known by only his name among most colleagues. His name is associated to being a dork/loser many times in the film. He just wanted people to notice him by hook or crook. So he imagined that he murdered many people and also imagined that a detective had come to interrogate him, as it would be obvious after the disappearance of someone (Paul Allen in this case). Or maybe the detective really came to Patrick. The detective (Willem Dafoe) says that Paul Allen was just missing, and throughout the film it is not said by anyone that he is dead, except for Bateman.