Clearly IxTJ. I'm thinking InTJ because I get more Ni than Si from him.
Check it out - dominant Ni goes into lucid fantasies of him going on murder sprees that he does not even know if he committed or not. Then again, of course you have scenes where he delves into textures (business cards) his various lotions (morning scene) he can remember all his appointments without writing them down (office scene) - this does point to Si, buttttt...I'm going to say that an ISTJ is probably not going to be unable to distinguish between reality and fantasy like that (okay, yes, he is psycho, but come on). An Si would be more likely to trace back and think how/why those may (not) have occurred, but he's in a daze so that we do not even know if he did them or not. And okay, he obsessively knows all about his favorite bands, but that might be a Te thing too???
Auxiliary Te to boot - that regimen in the morning? His plans on how to carry out his murders that may or may not have happened? The drive to succeed and be better (card scene, where he nearly freaks out because his business card is not cool enough). Okay, you could argue Fe with his ability to charm the hell out of the ladies and be polite when controlled, and he 'just wants to fit in.' But nah, Te is much stronger.
Tertiary Fi - he hates his surroundings, the senselessness, the debauchery, faces without names, no one with a backbone. He kills 'on the spur of the moment,' but does he really? Then why does he spare the secretary? It reminds me of Silence of the Lambs, where Hannibal does not kill Starling...sort of...though it does have a slightly different dynamic.
Inferior Se - enough to take advantage and kill spontaneously. Enough to react to situations with ease. To deal with things as they come. However, it's a shadow function - you only see it in him under stress (interrogation, killings). Otherwise he's very detailed, very deliberate, very planned.