The end rampage did not happen. Right? It was just too farfetched. Blowing up cop cars with single pistol bullets, and just shooting everyone in site. The "Feed me a stray cat" displayed on the ATM was also in his head, so why couldn't the murders be? Also, if he was REALLY storing bodies in Paul's apartment, wouldn't the so called detective have discovered them, considering Paul was the center of a disappearance investigation? I'm sure the detective would be visiting Paul's place frequently. But I think the detective was in his head too. Why do we never see him at the end?
Someone on imdb also mentioned the pills he took. He tended to take them before the so called "murders" and his murders were always related to something he has seen on TV. I say they were hallucinogenic. As I remember he was watching some horror movie with a chainsaw. Then he was watching a 3-some porn, both of which he reenacted later in the movie. There's probably more that I'm missing, but you get the point.
So yeah, I'm pretty strong on the murders did not happen theory.
However, one of the more intelligent posts at imdb (I know, it rarely happens) suggest that the viewer was meant to be confused at the end, and that this debate was meant to happen. You see, we are left with the same confusion that Patrick Bateman was at the end, and we don't know what to think. For all intents and purposes, we become Patrick Bateman.