- The show's dark tone is established with its first on-screen case: It's discovered that the wife of psychologist Dr. Mantlo, Amy Mantlo, is murdered by Peter Saldua, a mentally unstable man with violent tendencies caused by redheaded women (like Amy) and who is found later to have shot himself. But then it turns out that Dr. Mantlo wanted to get out of his marriage, but if he divorced Amy, he'd have nothing. So, he started to treat Saldua, give him steroids under the guise of medication in order to induce ''more'' instability via steroid intoxication, pressured Amy into changing her appearance to a redhead, and have Saldua deliver flowers to her (since he worked as a flower delivery man) in order to get him obsessed with her and murder her. Then, he faked Saldua's suicide. The fact that someone would do all this is twisted.
- Everything about the Balloon Man from "Child Predator", especially once you learn who he REALLY is.
- M aka Sebastian Moran. He ties his victims up, hangs them by their feet, then slashes their throats and lets them bleed to death. Sherlock states that M's youngest victim was twelve. It actually gets worse after the reveal that Moran is a hitman. Moriarty is directing him to kill people whose deaths are somehow vital to one of Moriarty's criminal schemes, and simply making it look like the work of a serial killer in order to throw off suspicion.
- Sherlock himself. During his little rant to Watson, he was somewhere in between Tranquil Fury and just plain crazy.
- The plan from "Red Team". Sufficiently plausible that the government classified it immediately; sufficiently terrifying that a member of the team was prepared to kill everyone else who knew what it was; deduced by Sherlock in a few moments. Of course, Sherlock would never reveal it... but what one smart man can figure out, another smart man can also figure out. The sort of smart man who might, say, casually employ an entire stable of serial killers to further his own ends...
- In "The Deductionist", the serial killer internet stalks blonde women over a certain height, and then skins them. Overlaps with Paranoia Fuel because people like that exist in Real Life.
- "A Landmark Story":
Moriarty's message decoded: Moran you never told me you had a sister. She dies or you do. Your choice. M
- Moran's killing of the guard simply for overhearing bits of his conversation with Holmes. And with such carefree glee afterward!
- Holmes visits Moran for help with a message he needs to decode from Moriarty. Moran claims he cannot translate. Sherlock translates it later:
- The Dissonant Serenity of Moran calmly singing the Arsenal FC anthem to himself in his cell before suddenly and violently bashing his face against the mirror.
- "The Woman": Can you ever really be sure about which of your loved ones is genuine and which is a carefully constructed façade who may not be the world's greatest criminal mind in your case, but still wants nothing more than your utter devastation?
- "Corpse Du Ballet"'s victim is a ballerina who is found in two pieces (she's split in the middle across the torso). The reveal of the body is very eerie, with the stage it falls onto partly shrouded in fog, meaning it takes a moment to realise that there's not two bodies lying there.
- In "The Great Experiment", Mycroft's crooked handler is trying to find out where he has gone, and threatens to gouge out one of Joan's eyes unless she tells him where Mycroft is. Fortunately she was prepared, but the way he switches from civil to coldly threatening to torture her with his bare hands is disturbing.
- In "The Five Orange Pipz", it is revealed that the killer was a corrupt FBI agent who wanted to steal the confiscated evidence before it was destroyed because it was worth millions on the black market. The evidence? Beads that turn into GHB when swallowed. GHB is often used as a Date Rape drug...
- In "The Illustrious Client" a beaten and barely dressed woman is flung into a room by a man largely believed to be the same man who had raped and tortured Kitty in London. Before he locks her in, he vows that her situation will worsen. Fortunately, the man's address is discovered and the police arrive to the woman's rescue. Then it is then revealed that the man, Simon de Merville, was working at an illegal brothel for sex slaves. The way the camera pans around to room after room where the women are held in is rather discomforting and chilling.
- In the follow-up episode "The One That Got Away", Kitty takes gruesome revenge on her rapist Del Gruner by burning his face off with acid. Granted, this is taken from canon, Gruner is definitely an Asshole Victim, and Kitty doesn't kill Gruner because Sherlock talks her out of it. It's still pretty disturbing.
- At the very end of "Hemlock", Joan's boyfriend Andrew dies right in front of her while she watches helplessly. The revelation that the poison was meant for her doesn't help.
- The apparent lack of security in the prisons of New York can be an example. The fact that two mastermind criminals were able to plot a murder, one whose victim was also still in prison and do so with no connection between her and the deed, while in jail does not speak kindly to how secure criminals really are.
- The car-app service used in "The View from Olympus" can come across as rather creepy - your every movement is tracked by your phone the moment you download the app. The workers of the company can track your movements, and figure out exactly what you're doing at every hour of the day, as seen in the episode, where one employee blackmailed two users for money, and another used the service to stalk a girl.
- Morland Holmes' reaction to an attempted blackmail. To wit: His contact in Interpol threatens to expose his (unspecified but sinister) plans regarding Sherlock if he isn't paid a significant sum. Morland, utterly unruffled, informs him that he had a previous Interpol contact who also decided their relationship was insufficiently profitable; apparently he met a gruesome end and was discovered by his young children. Still smiling, Morland offers to forget the conversation, as long as his contact leaves and does the same; if not, though... "How are your daughters?"
- Additionally, the fact that whatever these plans, he's concealing them so well that Sherlock, whose phenomenal powers of observation are only bolstered by many years' practice reading Morland, honestly seems to believe him to be genuinely attempting to belatedly forge a relationship, which is partially true, but with Morland, things are never just that simple.
- In "Art Imitates Art," the sheer amount of corruption that allows a district attorney to frame an innocent man for murder. Making it worse, when the case against the framed man threatens to fall apart, she kills an innocent woman with the intention of framing her brother for the crime, and for a minute, it comes close to working.
- In "How the Sausage is Made", the murderer grinds a man into sausages. The murder is discovered because one innocent person eats the sausage and dies of allergic reaction to horse tranquilizers used in murder, but who knows how many other human sausages were sold and eaten...
- In "High Heat" the staff of a crematorium open one of their ovens and discover it was used over the weekend when they notice an ashen handprint reaching for the door. The victim was stuffed into the oven and burned alive. Sherlock lampshades the nightmare inducing nature of the death when he passes a photo of the handprint to Joan.
- Season Six has Sober Companions, a nightmare reveal after reveal. For the police, it would have felt horrible to know that they failed in connecting several missing/murdered women cases to a serial killer. For the husband of one of the victims, he went to jail for a crime he did not commit, and even when Sherlock told him the police finally had reason to believe he was innocent, they did not yet have enough proof to release him. For Gregson, he realized the killer actually had contact with his daughter and her roommate. For Hannah, she spoke with the killer, almost went on a date with him, and found out the reason he couldnt make the date was because he was in her home killing her roommate.Holmes: This man caught me in a moment of weakness. Since then, Ive sat across tables from him, Ive been to meetings with him, hes been to our home. Im not going to let him take another life.
- And then the serial killer shows up right besides Sherlock when the latter is alone, suffering from a headache, and attempting to case the mans home.
- Then we have the penultimate episode of season 6, "Fit to Be Tied", where the serial killer returns from a hiatus. The ultimate nightmare fuel is when he catches Joan alone in the basement of the brownstone and starts beating her up, with the intent on murdering her like his previous victims. Not only that, but he catches her right as she gets off the phone with Sherlock. Joan had been investigating into the serial killer's friends, and her reveal that the serial killer had something to do with his staunchest friends' late husband's death, causing the friend to overdose on drugs infuriated him. Fortunately for Joan, she manages to stab him and drive him away, but she's still badly injured and the last shot of the scene is her crouching on the floor in pain and terror.
- Sherlock is notably subdued in the hospital, no doubt running through the horror of the situation over and over again that all of this started with him having allowed the man into his life in a critical moment of weakness.
Nightmare Fuel / Elementary