John Tagman is the main antagonist of the Law & Order: Criminal Intent episode "Want". He is a serial killer who turns his victims into mindless, subservient "zombies". He is based on serial killer Jeffrey Dahmer.
He is portrayed by Neil Patrick Harris, who also portrayed Foy in A Million Ways To Die in the West.
In "Want"
Tagman is a shy, quiet man who lives a solitary existence, with no friends or girlfriend. He has very poor social skills, so the love and companionship he desires constantly eludes him. He has a rich fantasy life, however, which grows darker and more violent as his loneliness grows deeper; he begins fantasizing about killing a woman so he can "possess" her forever.
's first victim is a stripper whom he drugged and strangled; he then removed her calf muscle and ate it. He then injects acid into his second victim's brain to so she will become his fantasy of a submissive, eternally beautiful partner who will never judge or leave him. She lives, but is left so badly brain damaged that she does not even know her own name.
Detective Robert Goren comes to suspect Tagman after seeing him on the strip club's security camera talking to the first victim. He and his partner Alexandra Eames visit Tagman's workplace, a chocolate factory, and notices Tagman's extreme shyness around Eames. He has Tagman detained on a false charge so they can search his apartment. They find cups of ice and a drill bit, as well as the second victim's calf muscle in a bodega across the street. They realize, to their horror, that Tagman has eaten part of it.
After Tagman is released, Goren takes him to lunch to "apologize", and profiles him; he sees that Tagman is a deeply depressed alcoholic with no social life, and theorizes that he feels remorse for his crimes and that he kills not for the sake of killing, but to create a partner who will never leave him. He empathizes with Tagman, and recommends to his skeptical colleagues that Tagman should be spared the death penalty and put in prison for life.
Goren and Eames arrest Tagman for kidnapping and assault, and trick him into revealing information that the District Attorney's office can use to convict and execute him. Goren has a moment of conscience, and tries to save Tagman's life by getting him to confess. He tells Tagman that he is sick, not evil, and that the only way he can be free of his pathology is to take responsibility for what he did. Tagman bursts into tears and confesses, which makes him ineligible for the death penalty. He is sent to prison for life, but is soon after beaten to death by another inmate.