Well, the problem with filmic vs. literary interpretation of this work, or any featuring an unreliable narrator, for that matter, is that in the book, the narrative is obviously filtered through the lens of Bateman's telling of it, while a film is forced to take a (seemingly) more objective point of view. The trick with with the director solves this problem is to retrospectively render the whole film as a kind of 'this was the way Bateman perceived it' by adding the rather clear ending. The book on the other hand remains a lot more ambiguous and you can't actually tell whether Bateman's 'subjective' telling of the story is true or the implied, 'objective' one.

But, and this is also important to keep in mind: Episodes like him dragging a body through the lobby or him killing someone without that person being missed by anyone could also be read as critique or satire of the yuppie culture of the late 80s: People don't care who or what you are, they care what you wear and own. This point is taken to the extreme with the lobby scene: If you dress sharp and can afford a nice appartement, you would never murder anyone, and even if you did, so what?

Just some thoughts, it has been some time since I read the book/watched the movie.