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Nightmare Fuel: Person of Interest
This show has comparatively rare instances of it, for all that it involves a Machine spying on all and sundry. But there are a few instances of it:
In universe, bombs are Nightmare Fuel for Harold Finch and if anyone he cares about is around one, he will immediately drop everything and try get that person of of the blast zone. The trauma comes from when he nearly died from a bomb blast which his Only Friend was killed in.
A lot of the stuff Kara Stanton says and does falls under this. Utterly ruthless inmore waysthe one, she seems to derive sick pleasure in causing as much pain and suffering as possible.
The Machine itself. Think about it for a minute; a computer exists that is capable of watching your every move and then uses the data it collects to decide whether or not you are a threat to society. Oh yeah, and it's owned by the government who have well trained and lethal hit squads at their command. Sweet dreams.
The newly liberated Machine is now arguing with Root about whether or not killing people is a good idea. Let's hope it doesn't lose the argument any time soon.
Root. Under that polite seemingly adorable smile lies a completely unpredictable Psychopathic Wild Card who's willing to commit horrible crimes in order to satisfy her twisted obsession with the machine. What's worse is that she's the second best hacker in the show who can find a dozen different ways to blackmail people to do her bidding and even Bad Ass former assassins like Shaw have been bested by her due to her utter unpredictability. If you turn your back to her, it's a high chance Root will take the opportunity to taser you, torture you and/or shoot you dead.
Andrew Benton; he's a serial sexual predator whose victims are too afraid to expose him, and at least one of them has committed suicide as a result. It's even more horrifying when you realize that the woman at the support group who gives a detailed account of being raped by an unnamed co-worker is one of his victims.
The episode "Proteus" is chock-full of it. No wonder, since it involves a ruthless identity-stealing serial killer stalking a few victims on a reclosed island. And of course it also had to take place during a storm, so we got that shot of the lightning illuminating the killer standing right behind Finch.
The scene where the killer talks to Harold is VERY creepy. He's talking about how he watched Harold all night, asks him "are you... Like me?" and calls Finch his "next great challenge" (taking over his identity). Then he takes Finch's glasses off, puts them on himself, and stares at Harold like he wants to eat him. The way he repeats "And what about you? And what about you? And what about... You?" Gave this troper chills.
"Nothing To hide" when you remove the Ass Hole Victim Wayne Kruger from the equation, the scheme Peter Collier instigates in order to destroy him is quite terrifying. Put it this way, someone you don't know and have never met is effortlessly revealing embarrassing and terrible things about you and you don't understand why or who is responsible. Then that same group of people tries to kill you by trapping you in an elevator and causing it to free fall and when that fails tries to cause a car crash. The icing on the cake is when the person who you thought you could trust then suddenly draws a gun and shoots you dead and the worst part is you don't fully understand why or what caused it to happen
"Razgovor": Root standing over Shaw with a taser as she's sleeping. For someone trained in intelligence and counterintelligence as Shaw is and after all the measures she taken such as destroying her cellphones after jobs with team machine, the fact that Root can still find her at her most unguarded moment is seriously creepy.
"Mors Praematura": Finch and the POI are showered in gasoline in a clever trap and nearly set on fire. Finch's terrified screaming sells it.
"The Crossing": Fusco's brutal torture at the hands of HR thugs. Made even worse when Simmons threaten to have his son killed over the phone.
What really makes it horrifying is Simmons chilling monologue on Fortune Cookies and near the end, Fusco's face is covered in a ghastly amount of blood.
A "Tall, Dark and Deranged" Reese who's in an incredibly self-destructive mood and beyond caring about the people who get in his way as he hunts for officer Simmons after Carter dies? It's a true demonstration of what "A real monster is like".
The Thousand-Yard Stare he gives as he leaves some criminals who gave Simmons a Canadian passport to burn to death in their car is a side of Reese only hinted at in "Many Happy Returns".
In John's episode Flash Back just before he kills the interviewer under the table, his head goes back into the shadows briefly forming the image of a skull
When he finally reaches Quinn, there's a chilling shot of his SIG-Sauer totally drenched in his own blood as he's on the verge of death
Simmons seeing Elias waiting in his hospital room. He can't disguise the fear in his eyes with his usual bluster, and frankly...fear is the appropriate response. End of the line, Simmons.
Root's torture by alternating uppers and downers, followed by Control removing her stapes without anesthesia.
"Death Benefit". The Samaritan under the control of Decima Technologies is now online. Imagine if The Machine had never been programed with the restraints Nathan and Harold built into it, allowing it to give a wider variety of information on a subject than just a single SSN and is able to be actively controlled by whoever is using it. And when said users happen to be a ruthless Private Intelligence Firm with even less scruples than the Northern Lights Counter-terrorism organization and with just as many resources, Team Machine is going to have their work cut out for them in staying alive. What's worse is that Greer has been able to convince senior members of the US government into allowing the Samaritan to become a potential replacement to The Machine
"Deus Ex Machina". Decima Technologies is triumphant, Team Machine has been utterly defeated and now Samaritan has set to work with anyone it classifies as a threat being eliminated, the first victims being the remnants of Vigilance who are gunned down by NYPD officers and Decima assets en masse using Samaritan to identify and eliminate them.
"I assure you it's quite the other way around. The question is what, my dear Samaritan, are your commands for us?"