I would like to discuss what could be causing Bateman to go crazy, it is one of the more mysterious (in my opinion) parts of the novel that I think Ellis leaves the reader to come to their own conclusions about. Starting on what I think may be causing his insanity, I think that it may have to do with his work. In the novel we see constantly that he has pretty much devoted himself to his work and in trying to be the best. In "The Shining" I remember the quote "All work and no play makes jack a dull boy" and I think that it is directly related to Patrick's situation. He is literally working himself to death, and when you work hard you tend to play hard, that might be why he is going to such extremes with prostitutes and murder.
I don't know about you, but I think that humans generally like to bring out violent emotions in forms of entertainment. That's why violent video games sell so well. In Bateman's case, he brings out his violent emotions when he is having sex (when he literally eats the one woman towards the middle of the book) because he has to restrain those emotions and appear sane at work.
This escalates as Patrick goes further along in the novel and he starts to have his insanity break out in his workplace, such as when he tries to kill his homosexual co-worker in the bathroom or when he yells at work, all up to when he spills the beans with his lawyer. This proves that his slip on reality was caused by his work because his work continued while he himself was slipping, sinking him deeper into despair and therefore directly increasing the activeness of his activities and violence of his murders.
One last note. Bateman's work is generally classy and specific. His aid is organized and knows how to handle herself. However I noticed that in Bateman's home he tries to make himself seem like his work, and therefore is bringing his work home without actually bringing it home. I think his inner self (unorganized, human, peaceful) is rejecting the work in his life and therefore brings forth violence because of his work.