Mark Lasswell in Commentary last year:

Up in Edmonton in the fall, though, Mark Twitchell did more than merely identify with Dexter Morgan. He wrote a movie script inspired by the series and then acted it out in real life. Posing online as a woman interested in a romantic liaison, he lured thirty-eight-year-old pipeline-industry worker John Altinger to a residential garage. And then, according to police, he tortured and murdered Altinger—just the way Twitchell’s hero Dexter Morgan would, because, you see, this most agreeable television character is also a serial killer. Twitchell was charged with first-degree murder and the script was seized as evidence. He pled not guilty.
Soon after the arrest, an Edmonton homicide detective named Mark Anstey said of Twitchell: “We have a lot of information that suggests he definitely idolizes Dexter, and a lot of information that he tried to emulate him during this incident.” At the time of the arrest, Dexter writer and producer Melissa Rosenberg was promoting the teenage-vampire movie Twilight, for which she had written the screenplay, but she soon found herself fielding questions from Canadian media about Twitchell’s affinity for her show. To her credit, Rosenberg did not adopt the usual Hollywood line of soberly contending that no one has ever shown a link between simulated violence and the real thing, a contention that is the studio equivalent of tobacco-company executives in Washington putting their hands on their hearts and claiming they had no idea that cigarettes cause cancer. The Canwest News Service reported on Rosenberg receiving news of the arrest and the Dexter connection: “‘Oh, Jesus,’ she exclaimed. She saw this as a ‘worst fears’ situation—something which had worried the show’s creators from the beginning.” Rosenberg insisted, though, that the series did not “glorify” Dexter Morgan’s murders:
Every time you think you’re identifying with Dexter and rooting for him, for us it’s about turning that back on you and saying: “You may think that he’s doing good, but he’s a monster. He’s killing because he’s a monster.”
The audience might be rooting for the serial killer because it is the particular inspiration of Dexter to make the character a responsible citizen who channels his murderous impulses strictly in the service of removing bad people from the world. Rosenberg said that the show’s creators had steeled themselves for criticism when Dexter made its premiere on Showtime in 2006. “The executive producers were expecting it. They were ready for it. They thought that we were going to get slams,” Rosenberg said, but there was “not a one.”
Well, here’s a one. Rosenberg had it precisely backwards, for just when you think Morgan is a monster, the show takes pains to ingratiate him further into your good opinion. Deviancy has continued to be defined down since the late Senator Daniel Patrick Moynihan identified the trend sixteen years ago, but Dexter represents a new low: the feel-good serial killer. The he’s-a-monster-no-he’s-not strategy of the show was apparent from the first episode, when Morgan abducts the director of a boys’ choir who moonlights as a serial killer specializing in the murder of his charges. Morgan has dug up some of the man’s victims and confronts him with the bodies—“Look or I’ll cut your eyelids right off your face”—before performing the ritual slaughter-of-the-guilty that is Morgan’s trademark. In this case, he goes to work on the man’s head with a power drill as a prelude to the butchering. “You’ll be packed into a few neatly wrapped Heftys,” Morgan patiently explains, “and my own small corner of the world will be a neater, happier place. A better place.”