Here is a movie with ambiguous ending and if you haven’t seen it stop reading here.
I like it when movies give the opportunity to the viewers to decide upon the finale. In our case Patrick Bateman is likely to have have done -most probably- two things: kill everyone that the movie portrayed or just everything was a product of his psychotic fantasies. Either way, it is quite amazing because, one, no one believed him when he confessed or, two, we were grossly deceived. I am voting for the second option.
American Psycho (2000) on IMDb.