*Joins this weeks book club*
If you read the book, it goes further into depth on everything. I haven't read it in a while, so I don't completely recall, but I seem to remember that Patrick Bateman actually did kill everyone, and his influence is what got him out of it. The "in his mind" thing was added by the movie. Not a bad touch though.
So you think his dad basically covered it all up? His influence comes from his father, at least in the movie, it said that his dad owned the company.