Dexter, a serial killer who preys on serial killers, used to be an emotionless flat-affect void like the type identified in Mark Seltzer's Wound Culture, motivated by bloodlust channeled into more or less acceptable channels by his police detective stepfather,
The "code" that Dexter's stepfather programmed into him when he discovered the boy exhibiting the early traits of a serial killer, specifically killing animals (he may have wet his bed and set fires as well to fit the classic pattern but the show does not tell us), assured that Dexter would only kill those who were a) likely to kill again and b) had somehow escaped the justice of legitimate law enforcement.
As the show has progressed, Dexter's character has developed, finding love and learning to be more human. But this season, in the wake of his wife's brutal murder, Dexter has turned into a mere vigilante, helping a would-be victim named Lumen (Julia Stiles) hunt down and kill the men who tortured her and tortured and killed 12 other women.
The men, who fall somewhere between the thrill-seeking, self-empowering white-collar amateur murders who were the clientele of Eli Roth's Hostel movies and the Sodality of Vice in Sade's 120 Days of Sodom, are led by Jordan Chase, one of those charismatic self-help guru who never seem to be anything but sadistic sexual monsters in the movies.
While the police, so incompetent in this season that weeks of investigating the man did not even turn up that he had changed his name in is 20s, are slowly closing in on the cadre of killers, but not before Dexter and Lumen have killed them all off, leaving only Jordan Chase, the most cunning and evil of them all.
Along the way, Dexter has hindered the police investigation that could have brought the killers to justice and closure to the twelve families whose daughters have been missing for years, killed an innocent (at least of murder) police officer who was trying to have Dexter arrested, and allowed Lumen to become a killer.
It must be said that Lumen did not need much convincing to become a killer. In fact, it is her presence, I would argue, that makes Dexter's killing morally problematic in an entirely new way. As Lumen and Dexter have become sexually involved as well as partners in crime, the ghost-vision of Dexter's father, who served as his "conscience," such as it is, has all but disappeared. In its place is Lumen, a shrill powerless victim whose obsession with getting back at those who have hurt her is far more distasteful somehow than all the violence in the show's previous five seasons.
Why? For one thing, Lumen does not seem to have come by her victim status honestly. We find out that before she was abducted, she ran a ways from her family and fiancée because the burden of "having her life planned out for her" was too much to bear. In her late 20s, Lumen found herself so exceptional, and her exceptionality so unrecognized by the people who loved her, that she had to flee to Florida to become whatever great thing she was so clearly destined to be.
What that thing was, apparently, was a victim. Abducted and tortured by privileged rich white men who played the part of upstanding citizens (of all TV and movie villains, the least problematic for audiences to hate) Lumen finally gained the exceptional status she had craved. Now, Lumen could place herself beyond all morality, utterly justified in killing her attackers—never mind that she almost shoots an innocent man whom she is positive was "one of them" on her first time out—and, like Dexter, a "special" being beyond the reach of common morality.
While the ring of murderers Lumen and Dexter are hunting are seemingly driven by a need to exercise with impunity the will to dominate that has made them such a success in their day-to-day lives, Lumen is driven by revenge, or more precisely, ressentiment.
There is nothing duplicitous in the murders committed by the evil men who abducted Lumen, they do not claim to be serving some greater good or victims of forces beyond their control, but she is dishonest on nearly ever level. She talks about justice, but shows no interest in it except when it is justice on her behalf. What she wants is to be like Dexter: above the law, without conscience, and able to achieve absolute mastery over the physically superior men who once subjugated her.
Dexter's motives have been so poisoned by Lumen's conniving ressentiment that he is prepared to kill the detective—not only an innocent, but a noble (in his own way) servant of the law—who can send him to prison for the crimes of which he is inarguably guilty. In the first season another detective discovered who and what Dexter was, but Dexter was unable to kill him because of his strict adherence to the code.
The presence of Lumen in Dexter's life and his utter abandonment of the code represents what is remarkably the first real morality crisis in the show. Dexter is no longer subject to his father's will or his twisted psyche, but to the self-justifying thirst for revenge of an embittered young woman who embraced the role of the victim long before she was ever abducted by the Sadean heroes who tortured her into insanity.
Simone Weil wrote:
"A man whose whole family had died under torture, and who had himself been tortured for a long time in a concentration camp; or a sixteenth-century Indian, the sole survivor after the total extermination of his people. Such men if they had previously believed in the mercy of God would either believe in it no longer, or else they would conceive of it quite differently from before. I have not been through such things. I know however, that they exist; so what is the difference?
I must move towards an abiding conception of the divine mercy, a conception which does not change whatever event destiny may send upon me and which can be communicated to no matter what human being."
Whatever its imperfections, the code was much more of a respectable morality than the thing that motivates Dexter now, the thing Nietzsche called ressentiment and Sartre "bad faith." And in the last episode, when Lumen's "dark passenger" disembarks and allows her to resume the fiction of being a good person while leaving Dexter to his grim reality, her bad faith is fully revealed.