He didn't actually kill anyone in the movie. He was crazy. He would imagine murdering people throughout the day. When he was imagining the murders, he would draw it in his day book. The only time he didn't draw it in his private day book was when he "killed" the hookers. In his head, he killed the girl with a chainsaw. The scene after the supposed "Murder" scene, Bateman appeared in a restaurant with his girlfriend where he was drawing a picture of a girl with a chainsaw cutting her in half on the table cloth. Meanwhile, his girlfriend was nagging him about random things which always makes him mad. Which can explain why he made such a big deal about the hooker being blonde. He was angry at his girlfriend (who is blonde), so, he was having a fantasy about killing her. The only reason why the movie didn't use the same actress is because it would have given away the movie completely. Another time the movie hinted that the murders were fake was at the very end when Bateman discovered that Paul, was in fact, still alive. During that scene, it showed small clips of his secretary discovering his day book full of drawings of people being murdered. The murders being completely imaginary also explains why Bateman was arguing with the chinese lady at the drycleaners. They were refusing to clean the sheets because they were perfectly clean already. He was imagining the blood.
Other things that support the theory:
-The Manager of his apartment building was totally oblivious to the blood left on the floor while he was dragging the body outside. Why? Because there was no body. Therefore there was no blood for him to notice.
-While he was putting the body in the trunk of his car, he was greeted by his friend. His friend paid no attention to his suspicious activity and was only curious about the bag itself. He asked Bateman where he got it. Which must mean that it was a nice bag. But how could a bag that trailed blood on the floor in the previous scene be nice? The blood must have soaked through, covering it in blood stains. The reason why it was still considered a nice bag is because there was no body in it and no blood. It was just an empty bag. That is why his friend had no reason to be suspicious.
-When Bateman was shooting at the police with his handgun, he blew up both police cars. This is obviously an unrealistic thing for a handgun to accomplish. So unrealistic that even Bateman was surprised. He looked down at the gun in confusion. Why? Because he realized how unrealistic it was for him to do that with a handgun. That whole situation was in his head which is why he was able to do such a thing.
-Bateman had unlimited ammo. In the scene where he blew up the police cars, he never ran out of ammo for his handgun. He shot an old lady, a few cops, shot the cars enough to blow them up, then he continued to kill a manager at a hotel, and shot a janitor. He did not have to stop to reload once. Why? Because he doesn't have to reload an imaginary gun. He never shot anyone.
-Nobody cared when Bateman either told them he was going to kill them, or when he told them about being a murderer. When Bateman was in the bar at the beginning of the movie, he told the bartender that he was going to kill her and play with her blood-- she completely brushed it off as if he never said it. When Bateman was at the drycleaners, he told the old lady that he would kill her if she didn't shut up and she didn't react in a scared way at all. Bateman also supposedly admitted to his girlfriend that he has homicidal tendencies and she didn't say a word about it. So obviously Bateman is imagining himself saying these things to people, so who is to say that didn't imagine the murders?
So my theory is that Patrick Bateman is just crazy. He never killed anyone.