I believe Bateman didn't really kill the prostitute on the bed.Earlier we see Bateman taking a prescription drug before he imagines/halucinates murdering Paul Allen with an axe.
We see Bateman pulling what supposedly is Allen's body in a travel bag past the doorman of the apartment, a trail of blood seeps from the bag, however no one reacts to this, not even the doorman or the colleague he runs into before dumping the bag in the trunk of a taxi.
Also in the very next shot this blood trail is gone (non existent), suggesting that this never happened and was all in his mind. Sometime before the scene with the prostitute and chainsaw Bateman is working out and The Texas Cainsaw Massacre is playing on the television (we briefly see this but this is important).
Before that he's watching girl on girl porn that he rented, there are times when he is questioned where his alibi or excuse is "I was returning videos" suggesting that he rents a lot of these movies (horror and porn) that play into his fantasies/psychosis.
Bateman already exhibits OCD behavior, an obsessive personality is already in play, his urges/psychosis begin to get the best of him. Hence prostitutes and escorts and chainsaws...the 2nd prostitute was so horrified cause in Bateman's mind she would be, it makes him powerful
Remember Bateman himself says he has no identity, all of these things (imagined murders etc) give him an identity,he is the big, scary and unbeatable monster/serial killer...to himself and to everyone else that he imagines forcing that onto.