I loved watching the American Psycho. I think it's a great movie that really focuses in-depth about the main character which many movies lack these days. But in MY point of view, the movie was made in such a way to spring many points of views.
Lets look at it this way, lets assume that Patrick Bateman (the main character) really is a high class citizen and is living a rich life. Becoming rich is not an easy thing to do, if your parents are rich in the other hand then it's quite easy to live this sort of life. But life does get quite boring on one hand, and there are a lot of rich people or any, that just piss you off. The way they live, the way they talk even the way they walk can lead you to think and wish horrible things for them.
When someone becomes wealthy or they have been wealthy their entire lives, they live and think in a certain way. They have certain standards and if these standards cannot be met, then they naturally get frustrated (like when Patrick couldn't book a reservation in that restaurant). What I'm trying to say is when all these things build up and you reach a point where you just snap, you either do things or imagine them. In my point of view they have structured this movie to be viewed from many different points of angle. It is your choice to either do it or not.