"So why do you think this show is so popular?" my long-suffering partner asked while we were watching Dexter. "Is it because Dexter is the classic outsider?" I shrugged at the time, but I couldn't get the question out of my head. Lord knows, she has a point. Dexter IS a classic outsider. Dexter Morgan is 95 percent of the appeal of the show. Subtract Dexter himself, and what you're left with is a run of the mill cop show, and not a very good one at that. I don't think Dexter's status as some kind of archetypal "outsider" is the primary element that appeals to the show's growing audience, though. I think it's more that Dexter represents the id. Dexter's id is the motivating force of the show's story and he even has a name for it. It's his "Dark Passenger." More broadly, Dexter is the patron saint of anyone who has ever had a terrible secret, for anyone who has had to juggle internal demons with external responsibilities. Like most popular entertainments that strike some kind of nerve, I think Dexter is a Rorschach test of sorts. What you get from it depends on what you bring to it.
As a personal note, I TOTALLY identify with Dexter Morgan. Not the serial killing part, mind you, but the need to keep some horrible dark secret and the need to fulfill some dark compulsion. I kept a secret of my own from everyone I knew--it was my own "Dark Passenger"--for literally decades, so looking at Dexter is like looking at a mirror for me, showing my own secret sharer through a glass, darkly.