Ok, I'd agree with Ni, definately. But I can't agree with the INTJ and here's why:
While he does exploit and use people, he is constantly driven by his emotions. His killings are random and uncontrolled. Not very INTJ like IMO.
Like I said, I'm also taking things from the book as well. He is most definetaly and I because he's constantly talking about how he's not really there when he's around people. He's completely inside and somewhere else. So that to me represents Ni as well.
Other than that, I would have said an E, but he seems so into himself. As I said earlier, if I wanted an example of an INTJ, I would say Hannibal. And don't forget, INFJs can mimic thinking types, so you can't always know. Hannibal would be listed as a sociopath who kills because he enjoys killing. Pat Bateman is a psychopath. Also, Bateman is conflicted within himself. He enjoys killing but at the same time tries to turn himself in. Another INFJ trait IMO.
But comparing and contrasting the two, I would still list Pat Bateman as an INFJ. But one that is very distressed and has a dark nature. I mean, come on, he is the American Psycho. lol
And if you still don't see him as an F......check out the romantic side of Patrick Bateman.
YouTube - American Psycho Redux