Was it all in his head? Was he some kind of super killer? Combination of the two?
Hello!?! There is no answer to this question folks! And that, of course, is the point and it accounts for some of the genius of the film.
Admittedly, I have not read the book. But comparing movies and books is almost always an exercise in futility and should be discarded from the outset. The movie should be judged on its merits as a movie. And this movie is great.
Aside from the obviously outstanding acting by Christian Bale (though Edward Norton probably would have been a better selection for the role), the film is great because it is heavily symbolic, dynamic, clever and very complex without being weighed down by these things. It is, for all its depth, still thoroughly engaging and can be watched without necessitating heavy analysis.
As a satire of the over the top 80s lifestyle it's brilliant, and as an examination of cognitive dissonance and the dark side of human nature/emotion it also scores a homerun. The self-reflective exploration of a radically image-conscious society is subtly reinforced throughout the film, both visually and in the dialogue. Consider, for instance, when Bateman emphatically declares to his fiancee, "I just want to fit in." This finds an interesting counterpoint in the hyperbolic nature of his lengthy monologues on music, which are so essentially meaningless -while striving to be meaningful- that they are rendered comical.
Another really clever aspect of the film is the opening montage; the credits. In the opening credits, you see a red liquid being splashed on a pristine white background. Aside from the aesthetic qualities of this segment, it also at first makes us think that the red liquid is blood. Indeed, we see a butcher knife flash, and given the title of the movie we are meant to believe there is something malicious and perhaps murderous underpinning this sequence. However, a few frames later we realize the red liquid is some kind of paste being used as a garnish and the knife is being used to cut meat.
This, of course, reflects the essential duality that struggles to reconcile itself throughout the film: reality vs. illusion. That's the kind of visual trick that can only be implemented in a film as opposed to a book. This is a very intelligent film.