This is extremely not how mental illness works (to the parody of seeming like a parody of mental illness conceptually) and I feel wrong not criticizing the film on that level, however at the same time I can't be too hard on it for that since it doesn't really set out to be an accurate portrayal of mental illness either. I don't exactly know what American Psycho sets out to be in the first place, actually, but if it's anything along the lines of a film that I enjoy watching, then it succeeded.