Garrett has one on full display just before he gets vaporized by Coulson.
The android AIDA flashes a very unsettling grin after she reads The Darkhold and constructs a portal device that saves Coulson, Fitz, and Robbie from being trapped between dimensions. At first glance one could assume that she was just happy she was able to rescue her allies, but this was the first time she showed a purely emotional reaction to anything, and it came after reading the demonic book. At the end of the episode, she's shown secretly drawing up plans for a human brain, and the unsettling smile is back...
The Big Bang Theory's Sheldon has a particularly disturbing smile that prompts his friends to say they're there to congratulate their friend, not kill Batman. (Howard: "Oh...crap, that's terrifying").
The Gentlemen of "Hush" wear these for the entirety of their appearance. And they never talk, despite gesturing at each other as if they were talking. The entire effect is ridiculously creepy.
The Gnarl, a skin-eating creature shown in "Same Time, Same Place", pulls this off chillingly. The smile is combined with a childlike personality, and it speaks in a singsong, like it's telling nursery rhymes.
Der Kindestod in "Killed By Death" who eats up kids' life force by pinning them down, extending horrible little pipes from his eyes and sucking the energy out of their foreheads, while they screamed helplessly under him.
The First does this as it's impersonating Cassie.
Angelus, Spike, Warren, and Buffy herself. She flashes a pretty smile to a group of bullies. Given some think she is crazy, others remember she burnt down her previous school's gym, and is on record for being a multiple murder suspect, is enough to make them back off.
Angel gives us Dana, whom Spike goes after, thinking she's a demon, and when cornered suggests having a talk about mistreating young girls and vamps out. Dana may well like to talk about mistreating little girls given her backstory, but turns out she's a Slayer, so when Spike does this her reaction is basically, "Score."
In his last story, Ainley's Master had fangs to boot.
Simm's Master perfected a dishonest politician's smile in his first episodes. After that, he got even creepier.
Gomez's Mistress (aka: Missy) gives the most amazing Slasher Smile to Osgood, too, complete with a creepy reveal from the shadows.
The Fourth Doctor generally sticks to the Cheshire Cat Grin, but he does this, usually when he's pretending to be evil or bluffing about how dangerous he is — like his I Am He as You Are He gambit in "Meglos". Or occasionally when he actually is being violent, like when he snaps a man's neck in "The Seeds of Doom". (To be fair, he only bent it a bit and the sound effect is just extra snappy — the man gets up soon after.)
"Dalek": The Ninth Doctor does this when he meets the alleged last Dalek in the universe. Thing is, its armour is completely shut down except for its voice and eyepiece, so what does the Doctor do but give a slasher smile and attempt to "exterminate" the Dalek. No wonder the Daleks refer to him as The Oncoming Storm.
The Doctor: I know what to do. I know what should happen. I know what you deserve! ..."Exterminate!"
The Twelfth Doctor already has a creepy smile, but at the start of "The Lie of the Land", when he is pretending to be brainwashed by the Monks, he gives a proper◊ slasher smile.
"Resolution": Lin gives a few of these while controlled by a Dalek recon scout. Normally Daleks are encased inside their armoured fighting machines, so this is a rare opportunity to show how much of a Blood Knight they can be.
(on hearing police siren coming up behind them) "I sense your thoughts. That sound is no rescue. It is combat!" (grins)
The Thirteenth Doctor is capable of delivering some pretty evil smiles when fighting her enemies, as seen here.
Scorpius of Farscape fame is infamous for these, accompanied by his black gums and needle-sharp teeth.
In The Flash (2014), Eobard Thawne becomes prone to these once his true identity is revealed. It's pretty unnerving, especially since as Dr. Wells he was quite stoic and soft-spoken, and had an understated but friendly smile.
Played for laughs in The Fresh Prince of Bel-Air. Will and Cartlon know that when Uncle Phil smiles and laughs uncontrollably, he's about to explode.
The Night King, though normally known for having an unreadable and chillingly emotionless face, twice makes this expression on the show. First when he nonchalantly walks through the Children's defensive fire completely unharmed; and later, he flashes Dany a positively chilling smirk after she unleashes the full might of Drogon's fire on him and he emerges unscathed. Note that both times the Night King makes a Slasher Smile is when magical fire fails to stop him.
Unsurprisingly, both versions of the Joker on Gotham are prone to do this, although it is considerably more unnerving when the one who usually smiles less does it. Unlike the show's first take on the Joker, the second character who takes up the mantel doesn't become as Laughably Evil as the character usually is until undergoing considerable Sanity Slippage. When he is first introduced, he hardly smiles at all, but when he does, it's nightmare-inducing, and usually means multiple people are about to die horribly.
In the British TV series Jekyll the main character's Hyde persona is equipped with a frightening one. It's currently the page illustration.
Kamen Rider Ex-Aid: When Kuroto Dan starts to occasionally flash an ear to ear grin towards the middle of the show, it shows that he has finally cartwheeled of the cliffs of sanity. The Ominous Visual Glitch usually distorting those shots underscores how close to a Humanoid Abomination he had become.
Cora gives some particularly disturbing ones while without her heart (i.e. most of her life). Even as she smiles and tells Regina she just wants the best for her beloved daughter, her eyes remain cold and lifeless.
Oz: Displayed among the characters like Shillinger, Adebisi, the Aryans, Timmy Kirk, Claire Howell, etc. Manipulative Bastard Chris Keller and Ryan have done this too. Even Beecher, when he was crazy in Season 2.
Reba: Van's "winning smile" is incredibly creepy, and he's completely oblivious to it.
Van calls it a "killer smile". Reba's description is a lot more accurate.
Reba: Van, that's not a killer smile, that's the smile of a killer.
Dr. Cox in Scrubs is very good at making these, a number of them occurring in J.D.'s Imagine Spots.
Miranda Hobbes on Sex and the City does one of these when she "fakes a sonogram" (i.e., fakes wild excitement during a sonogram of a child about which she's not quite happy, yet).
Mirror Chekhov: "So, keptin, you die... and ve all move up in rank. No vun vill qvestion the assessination of a keptin who has disobeyed prime Starfleet orders!"
A particularly lame episode of Star Trek: The Next Generation has Data practicing his dance moves on the Holodeck while wearing an extremely disturbing, frozen rictus-like smile.
Gowron, on the other hand, mastered this trope and showed it off in almost every appearance.
Strangers From Hell: Moon-jo's "smile" is truly horrifying. Especially when he's killing people or trying to convert them into serial killers.
Jessica from True Blood usually makes one of these whenever she is hungry and sees a guy she wants for dinner. However instead of being scary, it makes her look incredibly cute.
Twin Peaks: BOB, and by proxy Leland Palmer. His bared-teeth grin is the clearest sign that someone is possessed by BOB, as he usually only wears it right before he's about to horribly murder someone.
Word of Honor: Wen Kexing often wears an unhinged grin, especially when he makes sure other ghosts are too scared of him to rebel. It's usually followed by him killing someone.
An episode of Xena: Warrior Princess flashes back to the point in her past where Xena went from ruthless but otherwise sane and rational warlord to utterly psychotic rampager; she has one of these grins on her face when she snaps a Roman soldier's neck, but it's her eyes that are the most disturbing.
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