"How cheerfully he seems to grin! How neatly spreads his claws And welcomes little fishes in with gently smiling jaws!"
The Archonate: In The Spiral Labyrinth, which is set in homage to the Dying Earth setting, one of the significant villains is a wizard by the name of Smiling Bol. He always maintains a cheerful expression — sometimes less menacing, sometimes more so.
In the Brotherband book The Hunters, the author describes Thorn's smile as unpleasant when he's warning a captured pirate not to break for it.
Skeeter Traps from Chronicles Of Magic is known for sporting these, despite being a ten-year-old girl. Combined with the fact that she can kill grown men three times her size, it makes her pretty scary.
Jame, the heroine of P.C. Hodgell's Chronicles of the Kencyrath series, smiles like this, and it is very, very bad news. It signifies the depths of her darkness, and the author said, "In a way, everything about Jame grew out of that chilling smile."
From S.J Bryant's The Nova Chronicles, when Alden's attempt at turning himself into a monster fails, instead becoming a cute furry creature. Nova sports a wide grin as she brutally stomps the helpless creature to death beneath her metal boots.
Xellos from Slayers has this in spades. It's less common to see him not smiling than to see him with a smile (or a convenient Psychotic Smirk) on his lips, but if he's smiling and his eyes are open, you're in serious trouble. Especially exemplified as he's sadistically torturing Valgaav in TRY and a highly amused smile never leaves Xellos' face, backed up by a truly psychotic giggle.
As a Neat Freak, Ryuuji from Toradora! gets one whenever he cleans. As in, dirt and dust and mold. Combined with his permanent death glare, he looks like he's rejoicing in the thought of committing genocide on entire societies of dust mites and mold colonies. There's a brief shot of one right in the opening theme.
The Steel Inquisitors from Mistborn are exceptionally brutal enforcers of the Empire, and the chilling grins they have when going about their work are frequently remarked on. Anti-Hero Kelsier, leader of the rebellion, also smiles a lot because it's a way of proving that he's not beaten down, but from the point of view of his enemies (to whom he has little to no mercy) it comes off like this.
Bleeder in Wax and Wayne displays one of these while between forms, described as having "a face of stretched muscle and grinning teeth" and being "all smiles" as she scrambles towards her terrified cabbie.
Vlad Dracula in Count and Countess. Every time he's about to kill someone with his artificially sharpened teeth, he grins at them first, just to give them a preview of what's to come.
Sometimes it's a matter of perception. In Alan Dean Foster's The Damned trilogy, the first encounter between the multiracial alien alliance's and humans highlights the differences between them and us when the human, Will Dulac, first bats the gun out of the hand of the nearest alien scout out of a self-defense reflex, then offering a handshake and smiling when he realized they weren't hostile. The sudden reflexive strike nearly broke the scout's hand, and the smile confuses and alarms them as they wonder why baring one's teeth could ever be considered a friendly gesture. They also didn't get the handshake.
Deptford Mice: In The Dark Portal, a rat called Smiler had a Slasher Smile on his face all the time... because he'd been rude to his superior as a child, and said superior had cut off his lips.
Carcer Dun and Mr. Teatime are described as smiling like this all the time. The drawing of Mr. Teatime in The Art of Discworld perfectly illustrates this. Marc Warren does a good job with this in the TV adaptation, too.
At one point in Jingo the seriously pissed off "Vimes's grin was as funny as the one that moves very fast towards drowning men. And has a fin on top."
Dragaera: Perhaps inspired by the Jack Vance example, Five Hundred Years After has the court wizard as a minor character and he is similarly described as always smiling creepily and chuckling in a questionably sane manner.
In Jack Vance's Dying Earth series, the character Iucounu the "Laughing Magician" perpetually supports one of these providing a clue that his sense of humor is not exactly benevolent.
Creepy caretaker Heimertz from the Edgar & Ellen series has a constant slasher smile. During the TV adaptation, it is shown that he has had this smile since he was a child.
In Abnett's series Eisenhorn, the demonhost Cherubael sometimes sports one of these. When he/it does, someone is going to die, very painfully.
The Eschaton Series: Portia Hoechst, in Iron Sunrise, is described as having this by several other characters, both good and evil, in the novel.
A few Death Eaters display this, as does Snape on occasion and Voldemort in the Harry Potter series.
Benjamin, the serial killer antagonist of Hollow Places manages to torment one of his surviving victims while in court simply by showing her a disturbing smile.
Gods and Warriors: Alekto, the only female member of the House of Koronos, is so sadistic that she draws her lips back from her teeth in a way that's half a smile and half a grimace when she thinks about pain or better yet, observes it.
Randy gets an increasingly demented smile as he teaches Jeff a painful lesson in return for humiliating the former. What starts as a mischievous smile evolves into a less sane expression, which greatly concerns Keith and Troy, his own crew.
Jeff's outstretched smile before he kills Liu seems to be more out of pain than joy, but he wears the insane smile more naturally in the final chapter as he finds Randy to get his revenge.
The Icelandic Sagas: Older Than Print: Skarphéðinn Njálsson has one in Brennu-Njáls Saga. "And he grinned" is a common phrase in the story that implies that something really bad is about to go down.
In Jam everybody goes a little crazy during the course of the book but Cult-Leader Lord Awesomo has a memorable grin when he sees Travis in the crowd and realizes he can spin the election to his favor and get Travis killed.
The Jenkinsverse: Since most sapient species are herbivores and herd animals, they get very uncomfortable with human smiles, which involve far too many sharp teeth for their liking. Note that this is realistic; even on Earth among primates, humans are one of the only species that doesn't consider showing teeth to be a threat. Most humans who interact with aliens learn quickly to smile with closed lips. In Salvage, Adrian Saunders doesn't bother with that, which enhances his already terrifying reputation. When he finally meets more humans, it turns out that they find his smile even scarier than aliens do. At least aliens have little experience with humans and can assume that he's just normal. Humans know he's not.
In the Kate Daniels series, Kate is trying to enlist a man's cooperation with her plan to hunt down and kill the people who have hurt her friend.
I gave him a smile. I was aiming for sweet, but he turned a shade paler and scooted a bit farther away from me. Note to self: work more on sweet and less on psycho-killer.
Nathan in Kill time or die trying does this inadvertently; it's just how he naturally smiles.
Randall Flagg/The Dark Man is described as having a grin of such good cheer and jolliness which comes across however as so unsettling the human mind seems to block it out.
Also from The Stand (where King first writes about Flagg) is Harold. Most of the other survivors in Boulder just see him as a Stepford Smiler... but the slightly damaged child Leo sees things a little differently.
It's like there are worms behind his eyes, and they're eating his brain and making him smile like that.
The huge, catlike Kzinti of Larry Niven's Known Space universe instinctively show their teeth when they feel a certain way. That way is not happy.
Laughing Jack: Befitting his reputation as a hysterical killer clown, Laughing Jack's constant smile is a demented sharp-toothed grin. When he first catches sight of bloody violence in his origin story, he develops a small smile before it grows along with his escalating laughter, and he keenly watches the rest of Isaac's murderous activities with eager eyes and a big unfading grin.
Gwynplaine from The Man Who Laughs, as touched upon under the examples of The Joker and Mr. Sardonicus.
In Asi Hart's The Many Horrors Of Being A Tokyo Waitress main character Jonas has a very creepy forced smile until he is told that he's scaring the customers and instructed to use some other facial expressions.
Mr. Wonderful from Mogworld has this mixed with a Cheshire Cat Grin. It is described in the book as: "The widest grin he [Jim] had ever seen and he knew people with no skin on their faces" worth noting Mr. Wonderful's grin never drops even when he's hacking his own arm off.
The Name of the Wind: Cinder of the Chandrian has a smile that is described as "the expression a nightmare wore."
A rare heroic version: Ivarian Borenson, of The Runelords fantasy novels, is known for his chilling laughter and grin as he fights, which makes him feared even amongst warriors who have far more endowments (strength, stamina, speed, and so on taken from other people) than him. Not so much out of a love for killing, but because it works far better at frightening other people than a war cry, so he got himself into the habit of laughing instead.
Professor Moriarty, Sherlock Holmes' arch-enemy from the short story "The Final Problem." Holmes describes that, though Moriarty "smiled and blinked", there was something about the Professor's sunken eyes that made Holmes, a man who was used to danger, very glad to have a revolver with him.
Jaime Lannister of A Song of Ice and Fire recalls in his youth facing a particularly brutal and psychotic fighter known as 'The Smiling Knight'. 3 guesses why he is named that. Luckily even a slasher smile is no match for Ser Arthur 'The Sword in the Morning' Dayne and his greatsword Dawn.
In Neil Gaiman's Stardust, Lamia displays one of these during her search for the fallen star. Fortunately, we only have the Narrator's word on it.
Her red, red lips curved upward in a smile of such joy, such pure and perfect happiness, that it would have frozen your blood in your veins to have seen it.
In some of the X-Wing Series and any other appearances written by Aaron Allston, Wedge Antilles has often been known to smile like this — actually, while it can be assumed that he's got normal smiles, almost any time the narration says he's smiling it's this trope. Humorless, feral. He gives it to politicians when he sees through what they say to what they mean and most often to enemies he's about to shoot down.
In the same books, "Runt" Ekwesh (being non-human) has a facial structure such that even a friendly smile is described as looking like a prelude to a biting attack. He's keenly aware of this and takes full advantage of it when he wants to be menacing.
Legacy of the Force has Wedge shocked and taken aback by being discharged — Wedge, who was sixty and had been in service since the age of twenty — then realizing that people are going to try to kill him. The thought is so familiar that it steadies and reassures him, and he's able to flash someone a smile "suggesting that he was a rancor and they were made of meat".
The Storm (Arav Dagli): As the wife watches her abusive husband bleed out from her attack, a demented smile begins to crack on her face, followed by mad laughter.
Rictus in Clive Barker's The Thief of Always at first seems to be a harmless goofball who's always got a big, cheesy grin on his face — until the dark side of the Holiday House (his employer) makes itself known, and his smile becomes terrifying (if Harvey Swick had read his dictionary, he'd know that a "rictus" is the grin of a skull).
In Randall Garrett's "Thin Edge", Harry Morgan is confronting the man who ordered his friend's murder. While discussing the death of a man who tried to illegally search his hotel room while he was out, "Morgan did a good imitation of a shark trying to look innocent"
Similarly played in David Brin's Uplift War when an allied alien found it disturbing that humans bared their teeth and bark when amused.
Lijah Cuu from the Gaunt's Ghosts novels has one of these. In Straight Silver'', it is explicitly noted that "the most evil servants of Chaos would have killed to have a smile that lethal."
In the next book, Abnett gives Magister Enok Innokenti (one of the evilest servants of Chaos ) an even better one. His smile is so hideous that it causes physical pain and nausea in humans and his daemonic bodyguard, who has murdered millions can't bear to look at it.
The Hellebore family from the Tad Williams' book The War of the Flowers liked these. Anton Hellebore was described as having a smile like someone who'd learned it from a book, Lord Hellebore one with absolutely no good feelings in it, and the Terrible Child had one like someone pulling up the corners of a corpse's mouth.
In Warrior Cats, Mapleshade. She can make Ivypool feel like Daisy is with her, right before trying to drown her.
The Witcher: Geralt of Rivia is frequently said to be "smiling nastily", usually either to intimidate or show his supreme disdain for an idea or position that he's been put in.
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