In both the novels and the first television series, Dexter is a highly intelligent forensic blood spatter analyst who works for the fictitious Miami-Metro Police Department.

Consequently, Harry decides to channel the boy's homicidal urges in a "positive" direction by teaching him to be a careful, meticulous killer of people who "deserve it"—murderers who have escaped justice.

In the TV show, Dexter keeps his trophy blood slides from all his victims neatly organized in a wooden filing box, which he hides inside his air conditioner.

Dexter, in the TV series, eventually discontinues his practice of taking blood slide trophies during the seventh season, primarily because of the potential of him being caught due to a renewed secret investigation into the Bay Harbor Butcher, and because Debra has discovered his true self and he wants to be a better person for her.

Throughout the novel, Dexter is involved in the investigation of the "Tamiami Butcher", a serial killer who dismembers his victims and puts the body parts in refrigerated trucks.

Brian tells Dexter that as children, they witnessed their mother's brutal murder at the hands of drug dealers, and were locked in a crate for two days, surrounded by dead bodies.

Dexter persuades Brian to spare Deborah, and instead the two of them kill Lieutenant Migdia LaGuerta (this does not occur in the TV series, in which LaGuerta—whose first name is changed to "Maria"—is a recurring character until her death at the end of the seventh season).

Doakes and Dexter are soon forced to work together to stop "Dr. Danco", a psychopathic surgeon who removes his victims' limbs and sensory organs, leaving them in a state of living death.

By now, Dexter had embraced his role as stepfather to both children, but is annoyed when thoughts of them—wondering if Cody had brushed his teeth before bed and if Astor had set out her Easter dress for picture day at her school—distract him from hunting an intended victim.

There, while visiting an art gallery, Dexter and Rita see an avant-garde performance piece called "Jennifer's Leg" in which the artist amputates her own limb.

By the start of season one, Dexter is a blood spatter analyst for the Miami Metro Police Department; in his "secret life", he is a serial killer who preys on people who are themselves murderers.

Dexter has internal conversations with his late foster father, Harry Morgan (James Remar), who gives him advice about killing and navigating his personal life.

[14] However, during a therapy session with one of his intended victims, psychologist Emmett Meridian (Tony Goldwyn), Dexter is put into a state of deep relaxation, wherein he sees a frightening image of a small boy sitting in a pool of blood.

Facing certain death, Dexter resolves to keep fighting so he can live to see his son; he frees himself and kills the Skinner by snapping his neck and throwing his body into an arriving police car.

During the course of the season, Dexter justifies killing two people who do not fit his code: Nathan Marten (Jason Kaufman), a pedophile who is stalking Astor;[22] and his old friend Camilla Figg (Margo Martindale), who is dying of cancer and asks him to end her suffering.

[24] Once Dexter finds the killer, however, he is shocked to discover that "Trinity" is actually Arthur Mitchell (John Lithgow), a family man and pillar of his community.

[35] The eighth season opens six months later, with Dexter growing increasingly worried about Debra, who has quit the police force and spiraled into depression and self-destructive behavior.

Dexter discovers that Daniel set the fire himself to fake his own death, and that he is now living under the identity of Oliver Saxon (Darri Ingolfsson)—Cassie's boyfriend at the time of her murder.

[47] Approximately 10 years after the original series' finale, Dexter has moved to the fictional small town of Iron Lake, New York, posing as Jim Lindsay, a local shopkeeper.

He "falls off the wagon", however, when he impulsively murders Matt Caldwell (Steve M. Roberts), an arrogant stockbroker who once killed five people and got away with it thanks to his family's wealth and political clout.

[48] Over Debra's objections, Dexter once again becomes a father to the boy, who quickly becomes a popular student and star of his school's wrestling team, and befriends Angela's adopted daughter, Audrey (Johnny Sequoyah).

Dexter searches Harrison's room and finds a bloody straight razor—the same kind of blade that Arthur Mitchell used to kill Rita—and realizes that his son has inherited his homicidal tendencies.

[52] During a chance meeting with Angel Batista (now Captain of Miami Metro) at a police conference in New York City, Angela learns about Dexter Morgan's supposed death, and after some investigation discovers the true identity of "Jim Lindsay".

[53] Meanwhile, true crime podcaster Molly Park (Jamie Chung) arrives in town to do a story about Matt Caldwell, and Dexter discovers that she did a series on the Bay Harbor Butcher.

[53] When Angela finds the mummified corpse of her friend Iris, who disappeared 25 years earlier, she asks Dexter to analyze the remains, and tells him that she suspects Kurt of killing her.

Horrified, Harrison says that Dexter's "code" is a lie he tells himself to justify the murders he enjoys committing, and that if not for him, Rita and Debra would be alive and he would have grown up normal and happy.

Harrison leaves Iron Lake, and reads the goodbye letter Dexter wrote to Hannah 10 years earlier, telling her to "let me die so my son can live".

The San Francisco Chronicle said of the actor that "the allure of the series always has been and always will be Hall, who manages to make a killer (who kills only people who deserve it, mostly) likable, believable, engaging and funny".

[61] Commenting that Hall was "adept at portraying repressives", Ginia Bellafonte of The New York Times said this vigilante operates on the "stylized libertarianism that sees institutional failure wherever it looks".

[62] Calling Dexter "the thinking woman's killer", Wendy Dennis of Maclean's remarked that the show enjoys a high female audience because they are attracted to damaged men who are still sweet, handsome, and dependable.