After seeing this movie about a dozen times in the now almost 20 years since the movie came out, and reading the book somewhere in between, I love this movie for being able to be interpreted multiple ways. The most obvious way is that he is a serial killer but gets away with it.
Another interpretation that really popped during this most recent viewing was that it was all in his head... All of the scenes that make him out to be such a sociopath are over the top because they never happened (except in his head). He's a mentally ill, socially awkward guy who dreamed up the personality we see on the movie as his escape from his depressing reality. He is doing everything he can in an attempt to be mentally healthy; he works, takes care of himself physically, but he has no real relationships because he can't help it. He didn't kill any women, he unintentionally chases them away to the point even prostitutes don't want to be around him. Rather than have all those scarring memories as his crippling reality, he chooses to remember the people he chases away as victims of a smooth talking serial killer who we, the audience, can see is disconnected but he only appears disconnected because we're viewing him through a mentally ill person's perception of their own fantasy.
When he tells the bartender "I want to stab you to death, and play around with your blood.", that's just the reply he wishes he said but didn't. His breakdown at the end of the movie is his real cry for help because, in an attempt to live a "normal" life, he's been hiding symptoms of his illness too successfully for his own good and he's losing touch because of an illness he can't control. The drawings his secretary finds are real but aren't evidence of murder, they're just an example of his obsession with finding a girl to have a meaningful relationship with that he morphed into a Patrick Bateman that doodles these fantasy women as murder victims. The secretary of course will be further chased away by this, as has happened with everyone else he attempts to have a personal relationship with.