Patrick Bateman is a very misunderstood character. Admittedly he commits acts of violence so horrific that it is often hard to keep turning the pages of American Psycho, but his violent behavious is just a symptom of his inhuman condition. He represents the epitome of self-indulgence - the 1980s greed is good ethos taken to the nth degree. Instead of deriving an anti-moral pleasure from his killing he is rather an amoral individual who simply seeks to indulge himself on every whim. This inevitably leads to boredom causing him to escalate his behaviour.
His character is particularly fascinating because he is at once appalling and appealing, and the reader is both a spectator to his terrible deeds and his confidant. Were it not for the masses and masses of indescribably horrible violence he would be a rather charming but depressed young protagonist and a poster child for burned out 80's executives. In a way he is an empty vessel taught to consume and indulge, and it is the society around him that must be condemned for the results.