“American Psycho” ends with a confused Patrick Bateman wondering where the dead bodies went that he left in a victim's apartment and a lawyer telling him he had lunch with one of his victims last week. Meanwhile, back at Bateman's office, Jean comes upon his journal which is full of grisly drawings of his victims. This ambiguous ending is most commonly interpreted in two ways.
The first interpretation is that almost everything that happened in the movie was in Bateman's head. Bateman didn't really kill anyone, this theory states. He simply fantasized about it, and those drawings are his way of bringing those fantasies to life.
The fact that the bodies left in Paul Allen's apartment were missing, coupled with the drawings in Bateman's book tends to make people think the film was one long hallucination. Another scene that supports the theory is when Bateman shoots a cop car and it explodes, but cars don't really explode when you shoot them which leads people to believe it’s a fantasy.
The second theory is that everything that happened was real. Director Mary Harron supported this theory by stating, "One thing I think is a failure on my part is people keep coming out of the film thinking that it's all a dream, and I never intended that. All I wanted was to be ambiguous in the way that the book was.”
She continued, “I think it's a failure of mine in the final scene because I just got the emphasis wrong. I should have left it more open-ended. It makes it look like it was all in his head, and as far as I'm concerned, it's not."