Can someone please explain the ending of American Psycho to me in the simplest way possible, please?

I’ve read the book. Seen the movie twice. And I’m still lost AF.

Did he imagine the whole thing? Or did he kill all those people? Is it one of those stories that’s supposed to leave you hanging and guessing?

From the moment the swat team are after him and he calls his lawyer up and confesses - to when his lawyer thinks he was just joking and makes a remark about how Patrick Bateman would be too much of a coward to do those things. So is Patrick Bateman even Patrick Bateman or is his lawyer just getting smart with him?! Then his lawyer tells him he couldn’t possibly have killed Paul Owen anyway because he’d only had dinner with him in London just a few days before that. Yet Bateman had been indulging in a killing spree in Owens apartment all along? He had even visited the apartment to clean up the mess he’d left only to find a real estate agent there and the apartment looking all spick and span.

And then what’s with the “this is not an exit” sign at the end when he’s sitting with his cronies in the club?

Whew. I do love a good twist in a story but this one has always sprained my brain.