Toward the beginning of American Psycho Patrick is at the dry cleaners yelling at the workers because his bed sheets are still stained with "cranapple juice."
I've come to the conclusion that the murders were all in his head and that he had some sort of multiple personality disorder because Paul Allen called him Marcus and his lawyer did also
When he was dragging Paul Allen's body through the lobby of his apartment complex nobody said anything, there were traces of blood everywhere and the man asked him where he got the bag (if the murders weren't in his head it would just be symbolic of how upper class people are oblivious to things outside of themselves).
What about the bed sheets?
That's the only thing that is making me think that maybe he did kill people, because his old friend saw the sheets and seemed to be very curious about the red stains on them.