Subverted with the Cheshire Cat, who has this expression (it's significantly scarier than the Cheshire Cat Grin his other incarnations wore), but is one of the few allies the player has in the game, and he never attacks Alice. Still, when he appears and disappears grin-first, followed by his crazy eyes, it's prime horror. It's never quite clear whether he's as mad as a box of frogs, or frighteningly sane. Considering that the game takes the idea about Wonderland as a construct of Alice's mind to its conclusion by making Alice batshit insane herself, either possibility ends in horror.
Alice herself demonstrates one near the end of the game's trailer.
Batman Doom: Batman dons an evil smile after picking up the invincibility power-up or when he's in God Mode. He's also seen with a smile-like grimace on the title screen.
Bendy and the Ink Machine: Although the cartoon version of Bendy has a Cheshire Cat Grin that pushes his cheeks up into his eyes, the murderous 3D "Bendy" on the loose in the studio has this trope and ink completely covering his eyes.
Jin gets to sport one of these whenever he's around Ragna.
In BlazBlue: Continuum Shift, we have Hazama/Terumi, whose smile is perfectly fitting for his batshit insane personality. Especially on the Japanese audio track.
Buckshot Roulette: Adding to its frightening appearance, the dealer has a big, tooth-filled smile prior to being shot. It loses this afterwards, either because of the pain or because its mouth is simply too damaged to make one.
The Beast, a monstrous centipede from Bug Fables constantly has a wicked, fanged grin on its face. This trope, paired with its aggressive behavior paint it as less of a wild animal acting on instinct and more of a sadist who terrorizes and kills other bugs for fun.
Oblio of Dance Central when dancing "Body Movin" gives a smile that's just scary.
Doom: The Doomguy's face on the Expressive Health Bar gives a menacing smile when you pick up a new weapon, making it clear that he's excited to try out his new toy on the Legions of Hell.
The Yume Nikki fangame .flow has some of these, as shown in this walk-around video (video link) — for example, there are quite a few shark-toothed, bloody-grinning white blob things (whose faces are otherwise featureless) in the area first entered at 1:20, and the painting at 15:14 is of a surreal abstract line-art slasher smile.
There are also at least two places in .flow where the walls are made of those slasher-smiling white things, their mouths open even wider, and in one place it looks like their skins are strung up with the smiles still in the middle... even worse, the entrances to these places are the mouths of the white creatures. There's a video of these areas, too (video link) — one of the areas is entered first at 1:11, and more of it and the second area are shown starting at 7:40.
When the re-occurring character known as 'Smile' is struck with the steel pipe, he gets one of these. After the event occurs, Smile will grin every time you pull out the pipe in front of him... (for the morbidly curious, here's the recorded event: video link
In Dragon Age, both hurlocks and genlocks sport permanent Slasher Smiles and it's arguable that Shrieks also have one.
Found on one of the last fictional characters you'd expect in Epic Mickey. Should you decide to take the troublemaker path with Mickey himself, he'll begin to crouch low and sport one of these grins, looking all the more like a rat than a mouse in the process.
Sephrioth comes very close several times during Final Fantasy VII but manages to hold it back to simply a Psychoticsmirk, except in Advent Children he can't hold it back during his Resurrection and shows some teeth◊. In OVA Last Order he smiles mouth open wide to let out a Evil Laugh while wrecking Zack fair in mid air. In Final Fantasy VII Remake Seph puts aside any semblance of lofty restraint and cracks a deranged grinmultiple times.
Karel is an Anti-Hero with this. He's also something of an Ax-Crazy guy, considering he both constantly talks about loving to kill and he also murdered his whole family so he could get a sword. By the way, he looks like this◊.
Zephiel gives a nasty one at the end of the game, seen here◊. It's even worse in the leaked prototype version, seen here◊.
Ashnard, Mad King of Daein and main antagonist of Fire Emblem: Path of Radiance, is probably the definitive example.
Paxton Fettel has one of these plastered on his face almost constantly.
In Five Nights at Freddy's 2, the Purple Man is almost always seen with a smile when he's doing his...activities. "Almost" being when he's freaking out when confronted by the spirits of the children he's killed.
Friday Night Funkin'Game ModVs Whitty features Whitty with a grotesque rictus by the start of Song 3, "Ballistic." This is because he is uncontrollably pissed off with The Boyfriend and has gone insane with anger.
The Big Bad of Ghost Trick, Yomiel, wears one of these when he is first encountered. It's made especially unnerving by the fact that he looks identical to the main character Sissel.
It's the only portrait that Sissel doesn't share. Conversely, the only portrait Yomiel doesn't use is a goofy shocked face where Sissel's Triangle Shades turn diamond-shaped.
When Haseo awakens his Skeith Avatar in .Hack//G.U. volume one he's revealed to be wearing one after he summons Skeith's scythe and prepares to use it to slaughter Bordeaux and her lackeys.
Dedede of all characters manages a surprisingly creepy one in the intro for Gourmet Race. The lighting and angle he's seen at doesn't help in the slightest. It gets replaced by a silly Death Glare in the remake, thankfully.
Nightmare from Kirby's Adventure has a sinister grin full of sharp teeth. His Parallel counterpart has the same one in Super Kirby Clash. However, after going through his Villainous Breakdown and entering his Revenge form, this turns into a Slasher Scowl.
Sillydillo from Kirby and the Forgotten Land has a massive gaping smile on his face at all times. And while this is supposed to make him look silly as his name would suggest, the Maniac Tongue and ways of movement make this outright goofy smile look more deranged than silly.
The HeadThwomp, the second boss found in the Wing Dungeon in The Legend of Zelda: Oracle of Ages has this on its blue face. This boss has to be damaged when a bomb is tossed in its hollow top while it displays the red face, but if Link chooses another face other than the red one, it takes no damage and performs an attack. The attack it performs when the blue face is selected is firing spinning fireballs from said hollow top around the rotating platforms Link has to use to even reach it.
Link!!! of all people has one just after having a Nightmare Sequence has creepy dreamy smile appear◊ on his face when the dark thoughts take over his mind, but he snaps out of it and drops to his knees horrified.
Ganondorf in the flashbacks of The Legend of Zelda: Tears of the Kingdom is initially shown either frowning or sporting a low-key Psychotic Smirk. But when he kills Sonia and takes her secret stone, his lips pull back into a gleeful smile upon realizing that his Evil Plan is coming to fruition.
Ax-Crazypre-Heel–Face Turn Maverick Zero, as seen before the climax of Mega Man X4. And fitting, since he's built for the sole purpose of destroying the world.
Bass in the original series mostly looks like The Stoic. When he does smile, however, it's invariably either a Psychotic Smirk or this trope. Behold!◊
Hot Coldman, in Metal Gear Solid: Peace Walker, constantly smiles rather psychotically during his appearances. In fact, outside of being the man who set The Boss up or The baldie, probably the most recognizable characteristic of his is the smile.
The fake Colonel from Metal Gear Solid 2: Sons of Liberty, during his conversation with Raiden nearing the end of the game, occasionally gives off a Glasgow-esque grin (akin to Heath Ledger's Joker character or Dissidia's redesign of Kefka Palazzo.). Justified, as during these occasions, the Colonel's face often takes a form resembling a skull.
Jetstream Sam's introduction scene in Metal Gear Rising: Revengeance comes with one of these. It just looks wrong on several levels, but this is probably intentional to add an additional sense of unease to him.
Raiden also sports several in the same game, after reawakening his Jack the Ripper persona.
In Metal Gear Solid 3, Colonel Volgin flashes quite a lot of these, usually before cheerfully committing acts of depraved cruelty and evil. His most memorable slasher smiles occur when he nukes his own country just for the hell of it, when he sees that Snake has lost bladder control because of how intensely he's being tortured, while saying "Yes! Yes! ''That's'' what I want to see!" His other most memorable one is when he controls the Shagohod, and, while laughing maniacally, mows down his own men, again, seemingly just because he can. A lot of these traits are why Volgin is one of the few villans in the Metal Gear franchise to have next to no fans who sympathize with him, because he has no Freudian Excuse, no underlying issues, other than an extreme and insatiable sadism.
Baraka from Mortal Kombat perpetually has a deranged, fanged one that distorts his entire face. This, along with his glowing cat-eyes, can be a bit distracting to some players.
Mileena has a similar smile on her face, one that's usually hidden behind her ninja mask.
Moge-ko from Mogeko Castle sports some truly terrifying ones sometimes, especially when she's thinking about something particularly sadistic.
Jackle in NiGHTS into Dreams… is constantly wearing one plastered over his otherwise invisible face. You kind of get the picture that he's absolutely insane, because just adding that evil cackle... *shudder*
Matt Engarde shows a particularly disturbing one when you make him show his true colors in the second Phoenix Wright: Ace Attorney game.
In InvestigationsShih-na gets a bit of one when unmasked as one of the villains before eventually going on to a full-blown Higurashi: When They Cry-style Evil Laugh.
As does Manfred von Karma, although it's only slightly creepy unless you know that he murdered Edgeworth's father. AAI doesn't specifically spell it out, most likely to avoid spoiling those who haven't played the original game, but to those in the know it's creepy.
In Ratchet & Clank: Up Your Arsenal, CourtneyGears gives out one of these right before her boss battle. Just before, she had captured Clank and replaced him with a duplicate to spy on the Q-Force. She also abducts Skidd McMarxx, and turns him into a robot; when Ratchet finds out, she captures him and spills out that she intends to do the same to him in sadist glee.
The cover for Shin Megami Tensei IV has Walter sporting one of these, and Issachar also suddenly sports one of these when he begins to turn into a demon.
In Shin Megami Tensei IV: Apocalypse, Flynn, the saviour of Tokyo and Mikado, sports a very wrong-looking one of these while fighting Merkabah and Lucifer in Camp Ichigaya. It's a warning sign that it's actually Shesha disguised as him.
The protagonists of Persona 3, Persona 4, and Persona 5 frequently get one the first time they summon their Persona. The film adaptation of Persona 3 makes it even more prominent... and the gruesome summoning method of shooting himself in the head makes it particularly striking in both the game and movie.
In Persona 3, the final boss sports a mask which is nothing but a pair of hollows for eyes and an immense grinning mouth; this is in contrast to other Major Arcana Shadows, whose masks are either inhuman or emotionless.
In Persona 4, the real Killer, Adachi, sports these after being found out.
While the normal in-battle appearances of the P3 and P4 cast are retained in Persona 4: Arena Ultimate, each character has a Shadow version that invariably sports this or a Psychotic Smirk in their splash artwork.
Persona 5: Joker, Skull, and Mona flash these pretty often, Fox gets a savage one right before his first battle, and pretty much all of the bad guys show this off at one point or another, with the creepiest being Cognitive Akechi from the seventh Palace. Of the remaining thieves, Panther and Queen face enemies with scowls while Oracle and Noir only go as far as Psychotic Smirks.
The smiling green snake in Slither.io, especially when it's circling you or devouring your remains.
Rajan shows one in Sly 2: Band of Thieves before the title card for "The Predator Awaits" episode, (and for the picture for the chapter) Dimitri shows one to a lesser extent at the end of his intro scene, along with crazy ass eyes.
Penelope Mouse in Sly Cooper: Thieves in Time also sports this at the climax of Chapter 4 when she steals Sir Galeth's cane. This solidfies her as a heartless sociopath and yandere who willingly crossed the Moral Event Horizon, and the absolute proof to the Cooper Gang that she used them for her selfish desires, and was never on their side at all.
Red Savarin of Solatorobo gets an absolutely crazy one when The Order is given. His usual upbeat, happy-yet-stubborn demeanor means it's even creepier.
The little ghosts that appear mostly in Pumpkin Hill, and dotted around other stages in Sonic Adventure 2 each have a rather creepy, fanged, smile.
Strange Flesh: The Bartender is normally The Stoic, but his transformed demon form after you defeat the final boss sports a very twisted grin.
Normally, Akuma in Street Fighter is a Perpetual Frowner. On the occasions he becomes Shin Akuma, however, his face twists into a horrific grin.
M.Bison (Vega in Japan) almost always sports a slasher smile that is wide enough to show all of his teeth and even the gums. In his case, this expression overlaps with Perpetual Smiler.
The not-so-well-known variant of Ken, known as Violent Ken, has one permanently plastered on his face. His altered nonchalant sounding voice and relaxed fighting stance make him even creepier. His best attack involves screaming like a psychopath and panting heavily at the end just to seal the deal.
Vega (Balrog in Japan): nothing breaks that Bishōnen face better than the smile◊ he cracks when he activates his V-Trigger; no matter how beautiful and delicate he claims to be, Vega is easily one of the most insane Street Fighters.
In the Yoshi's Island games, Fooly FlowersOriginal nameDizzy Dandies are enemies that resemble the collectible Flowers, but their expression is an unhinged grin instead of a kind smile. When Yoshi approaches them, they drop to the ground and quickly roll towards him, hurting him upon contact.
King Boo sports a permanent grin in almost all of his appearances, but only in the Luigi's Mansion series does it cross over from a "mischievous troublemaker" smile to a "just thought of a fun new way to murder somebody" smile.
Alice. The way she occasionally switches from her typical smiley expressions to her evil-/scary-looking one in a single instant is so creepy it makes her bear an uncanny resemblance to a certain aforementioned mask salesman...
Alexei from Tales of Vesperia displays one in his cut-in just before the climax of his Brilliant Cataclysm mystic arte.
The Horseless Headless Horsemann from Team Fortress 2 has one of these carved into his jack o' lantern head, as well as on his huge ax.
Sweet Tooth occasionally flashed a malicious grin, especially in Twisted Metal 4, but starting with Black,, he wears a similar looking clown mask with permanent slasher smile.
Calypso takes glee in purposely screwing up the wishes of the contestants (in often fatal ways), so it's no surprise that his evil grins can vary from Psychotic Smirks to grins that can go from ear to ear like in Twisted Metal 2.
In Undertale, Flowey the Flower's normal smile turns into an enormous toothy grin that takes up almost his entire face when he reveals his true colors. It's especially unnerving when he becomes Omega Flowey in the final battle of the Neutral ending.
Goblins wear these in the Warcraftseries. The Goblin Shredder pilot has a particularly mad grin.
Some villains in World of Warcraftwere notable for these, including Dar'khan Drathir and Kael'thas Sunstrider: both power-mad blood elves. Since elves have fangs, their smiles quickly cross over into croc smile territory.
One of the creepiest features of Illidan's character design is that you can't ever tell whether he's doing this or actually smiling, since his eyes are covered. Since he's an Anti-Hero, he does both.
In the Mists of Pandaria expansion to World of Warcraft Blizzard flexed their animation abilities by having one of these occur on a member of the cutesy Pandaren, named Taran Zhu. It even gives Garrosh Hellscream pause. He didn't expect the peace-loving monk to insult his father or give a creepy grin while doing it. Taran Zhu is also a rare completely heroic example, a Snark Knight who just likes to creep out his opponents.
Two bosses in Wario Land 4, Cractus and Spoiled Rotten, twist their faces into grotesque, rabid grins after they've been hit several times. The freakiness is magnified by the fact that their heads alone take up more of the screen than Wario does.
The citizens of Wellington Wells We Happy Few smile almost constantly, even when they're trying to beat you to death for being a Downer. The most extreme example is probably the Doctors, who will come after you if you fake taking Joy and have big, unnerving rictus smiles on their faces at all times.
Certain levels in WET has a random mook charging Rubi, who promptly shoots them. Blood sprays onto her face, which triggers Rage Mode and she wears a psychotically evil slasher smile throughout the level.
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