How can the chainsaw-scene of "American Psycho" (2000) be explained? [duplicate]

The filmmakers left it ambiguous.(Atleast they wanted it to) You can interpret it as you like. Its all up to the viewer. It is a mind bending movie and the chaos brings in the marvel . Is it real or is it all just his imagination??!!!

The Director herself agrees that she failed to keep the film to be ambiguous in the way that the book was.

Mary Harron says that: "I think it's a failure of mine in the final scene because I just got the emphasis wrong"

As we can see the climax scenes just got too much unrealistic. They just failed to bring in the balance.

What The Filmmakers Say

In what can arguably be seen as the biggest twist related to American Psycho's ending, writer/director Mary Harron and co-writer Guinevere Turner have actually gone on record saying that -- in retrospect -- they don't love how the film's ending came together. This is because they feel that the finale is actually too ambiguous, and they don't love the fact that people walk away from the story wondering if it's all just a dream. They want to make it very clear: Patrick Bateman is most definitely a serial killer.

It was while being interviewed by Charlie Rose a few years back that Harron addressed divisive opinions about the end of American Psycho, and she explained that it was never her intention to try and get audiences to rethink the death and murder that had occurred throughout the film. Instead, the problem was that she feels she was unable to match the uncertainty in the ending of the original novel, and didn't make the movie's point clear enough. She explained,

One thing I think is a failure on my part is people keep coming out of the film thinking that it's all a dream, and I never intended that. All I wanted was to be ambiguous in the way that the book was. I think it's a failure of mine in the final scene because I just got the emphasis wrong. I should have left it more open ended. It makes it look like it was all in his head, and as far as I'm concerned, it's not.

You can watch the extended interview -- which is a roundtable with Mary Harron, author Bret Easton Ellis and Christian Bale -- Click Here

Courtesy: www.cinemablend.com

Also See: The One Line In ‘American Psycho’ That Will Completely Change The Way You See The Film - A very interesting interpretation by Adam Pliskin

I believe Bateman didn't really kill the prostitute on the bed.Earlier we see Bateman taking a prescription drug before he imagines/halucinates murdering Paul Allen with an axe.

We see Bateman pulling what supposedly is Allen's body in a travel bag past the doorman of the apartment, a trail of blood seeps from the bag, however no one reacts to this, not even the doorman or the colleague he runs into before dumping the bag in the trunk of a taxi.

Also in the very next shot this blood trail is gone (non existent), suggesting that this never happened and was all in his mind. Sometime before the scene with the prostitute and chainsaw Bateman is working out and The Texas Cainsaw Massacre is playing on the television (we briefly see this but this is important).

Before that he's watching girl on girl porn that he rented, there are times when he is questioned where his alibi or excuse is "I was returning videos" suggesting that he rents a lot of these movies (horror and porn) that play into his fantasies/psychosis.

Bateman already exhibits OCD behavior, an obsessive personality is already in play, his urges/psychosis begin to get the best of him. Hence prostitutes and escorts and chainsaws...the 2nd prostitute was so horrified cause in Bateman's mind she would be, it makes him powerful

Remember Bateman himself says he has no identity, all of these things (imagined murders etc) give him an identity,he is the big, scary and unbeatable monster/serial killer...to himself and to everyone else that he imagines forcing that onto.