Why does a modern television audience sympathize with Dexter?
Our leading man is a serial killer. He constantly obsesses and dialogues with his dead adoptive father. He keeps blood slides and murder weapons and consistently deceives the people around him in order to rack up body counts. The show's very premise inspired protests from groups such as the Parents Television Council. We see Dexter's mania manifesting virtually every week.
Consider this week's episode: "I need to kill him," Dexter Morgan says. He pants, holds a knife, stares down at an ex-con's body. The vision of Harry chides Dexter and points out the holes in his logic. Dexter stops but is furious. "I was ready to kill him!" he repeats in frustration at his mistake as he violently pulls down the plastic sheets. He realizes the man wasn't responsible for the abduction of the young woman named Lumen Pierce (Julia Stiles). The scene chills precisely because it reveals the psychological damage motivating the show's title character, whether in the form of voices or his willingness to inflict lethal violence.
The impulse in Dexter is, despite the graphic and bold extreme of the character, not culturally unfamiliar. It's the impulse of the anti-hero, the vigilante, except with all the sanitary super-hero cosmetics stripped away. Forget his psychosis and remember one critical tenet justifying the show: Dexter kills bad guys. His kills reflect a wild righteousness, a sense of code-driven vengeance. Aren't these victims deserving? Virtually all had been killers themselves. All of Dexter's kills aim for the same metaphorical target—the men who murdered his own mother (a la Batman with his own parents). Dexter ultimately saves lives, and as he imagines at the end of the first season, perhaps the crowds should really be toasting him for his work. It's the catch that allows us to forgive what appears on screen as unrepentant psychosis on par with legendary killers from Ted Bundy to Dostoevsky's fictional Raskolnikov. The complicated morality scales place Dexter on the side of Superman, not Jack the Ripper.
Lumen, our new character in season 5, is motivated by that same morality of lethal vengeance. Her abduction and rape justifies the murder of her abductors, she reasons, and nothing will stop her from seeking out the guilty parties.
Dexter hates the idea for two reasons. First, as ghostly Harry intones about Lumen: "Does this seem like the product of a stable mind to you? ... Dex, she's gonna bring you down." Secondary is a reluctance to accept his own violence and morality, a recognition of the trouble and risks it brings. Lumen needs to stay away from the darkness, Dexter thinks. I, on the other hand, always need more names for my table. The cute and genuine concern for his son Harrison's playground violence reflects this same reluctance. He wants to prevent anyone else from feeling a compulsion to his brand of justice.
As Dexter well knows, the compulsion is tied to one's own psychological damage. His righteous killing is an addiction, as the show cleverly vocalized in Dexter's second-season Narcotics Anonymous attendance. No choice exists, and the compulsion earns audience sympathy. Lumen shows the same signs of damage: the way she picks apart her sugar packets as she talks, her hands shaking on the gun, the panting flashbacks at the airport security check. Audiences can appreciate such vulnerability. Anger is a justified, correction emotion; the characters are right to want vindication. Dexter and Lumen internalize the crimes against them, and their sense of being wronged makes them helplessly compelled to act.
And that's also just the problem, really—such underlying trauma is what will doom any of these vengeance campaigns to sloppy emotion. Hasn't that sloppiness defined Dexter himself for the past season and a half?