We now have a cable television subculture saturated with antiheroes. Walter White on “Breaking Bad,” Tommy Gavin on “Rescue Me,” Patty Hewes on “Damages,” Nucky Thompson on “Boardwalk Empire” and, to a certain degree, Don Draper on “Mad Men.” Add to that list Dexter Morgan, the titular “hero” of this Showtime series and you have one hell of a party.