As far as the most devious and subtle of villains are concerned, when it comes to facial expression, less is more. A Cheshire Cat Grin only warns people that something's afoot; a Slasher Smile is even more glaringly obvious. No, when someone's about to walk straight into your Death Trap or your Evil Plan is about to pay big dividends, it's best to show your smug superiority with a devious little smile, probably behind your victim's/lackey's back, or behind your hand (or hands). Only the villain and the viewers know that alarm bells should be ringing in the good guys' heads - but hey, they're the enemy, why give them advance notice? For obvious reasons, this subtle hint of treachery is especially likely to turn up as part of a Traitor Shot.
The Psychotic Smirk is a favorite with Big Bads everywhere. There's no need to broadcast just how dangerous they are - everyone's got a pretty good idea already, but no one can prove it. The important thing is to smile with only one side of the mouth. Sometimes, it's accompanied by a Finger Wag as if to say "Nice try, sucker" if the hero had attempted to defeat them.
Unfortunately, the understated effect is often ruined by an over-the-top Motive Rant when the penny finally drops for the heroes.
A particular type of Anti-Hero does this, too: the soft-spoken, underhanded ones who always know more than they let on, but never see fit to warn others of the danger.
See Sideways Smile for the generally less evil version.
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Anime and Manga
Alucard from Hellsing almost always has this, so much so that it's hard to distinguish this from his Slasher Smile.
The Major also sports one quite a lot. Pretty much everyone in the series has had one.
The Demons of Ah! My Goddess: Velsper and Hilde more so than Mara, who's a little too over-the-top to pull off the "subtle mastermind" shtick.
Anti-Hero type: Count D in Pet Shop of Horrors, particularly when he's manipulating a presidential candidate and his aide in Dual and when he tells a couple to "take good care" of their new pet rabbit/surrogate daughter in Daughter. In the anime, this is made even more creepy when the darkness of the screen emphasises his smirk and his mismatched, glowing eyes.
Xellos from Slayers is also an antihero version. Of course, he smiles all the time, so his smirks are definitely important when they show up, usually complimented by a slight opening of his eyes, another important visual cue.
Noa, villain of the Virtual Nightmare arc of Yu-Gi-Oh!, has this expression when watching the misery of the heroes from the comfort of his cyber-throne. He gets it almost perfectly right.
Yami himself. An Anti-Hero example of this and he got it perfected.
Naraku from InuYasha typically has this expression when he has the upper hand and isn't trying to trick the heroes into thinking he doesn't (which is pretty much always).
Legato Bluesummers from Trigun puts off several of these.
Headon from Tower of God at Baam's decision to take the test and climb the Tower. As we know now, it's because Baam unintentionally set off the final stage of the age-old conspiracy against the Tower's king.
Gin Ichimaru from Bleach, always seems to have an enormous grin on his face. If that's not enough to give him away, his constantly slitted eyes will probably do it.
Aizen. If you see him smiling even a bit, turn heel and run, 'cause you only have seconds to live. The only problem is that, he's always smiling.
Light from Death Note. Three words: "Exactly as planned." One smirk he busts out is so momentous, it's accompanied by a sudden earthquake in the cinematography.
Also, Mello in the scene where his henchman is killing much of his opponent Near's team. In the anime, in the middle of the scene, while the dice are falling and the people are dying, it cuts to Mello, and a satisfied grin. Also chocolate. Always with the chocolate.
It's never certain whether Gendo Ikari is actually the Big Bad of Neon Genesis Evangelion, but this is an important facet of his character - every time you see the EVA units going into action, the camera cuts to Gendo seated at his desk, with a smile hidden behind his folded hands.
Lelouch Lamperouge in Code Geass loves doing this behind that Cool Mask of his. For good reason; whenever he does it without the mask it passes from awesome to terrifying.
We're introduced to that smile at the end of the first episode, when he grins just a little too evilly after Geassing the soldiers about to kill him into shooting themselves. It's the blood splashing onto his cheek that really does it.
Kazuma from Kaze no Stigma. He really doesn't like the Tsuwabuki family...
Itachi from Naruto seems to consider this his favourite expression. Of course, he only has three; frowning, blank stare, and smirking. If he smirks...you're in trouble. Direct, immediate, trouble, that is. Most likely he's just reversed your attack and you're about to kill yourself unless you stop.
Suigetsu does this most of the time, usually one with his mouth closed if he's genuinely indifferent, or with it slightly open (showing off his shark teeth) if he's about to kill someone face an opponent he thinks he can easily beat. He usually only does otherwise if he's in some sort of danger.
Kyuubi!Naruto sports an amazing one near the beginning of his fight with God Realm Pain. It's the middle right panel on this page. You don't get better Psycho Smirks than that.
Now Obito has jumped in on the action after being unmasked. His response to being asked if he really is Kakashi's old friend is just a long parade of these one after another.
If Akito from Fruits Basket (the manga, at least) smiles, this is usually the smile he chooses. Of course, if you take in what his eyes usually say ("I am so enjoying ripping your little psyche to shreds!"), it's a little more disturbing...
Vegeta's psychotic smirk was one of his trademarks as a villain, given just before blasting someone to Hell (or the Next Dimension, for those of you who go by the earlier dub).
In Episode 37, Kimblee actually has a very sweet smile when he tells Winry about her parents, and how his squad was too late to prevent their demise at the hands of Scar. Which only gets just as disturbing once you learn he and his squad were on a mission to dispose of said parents for assisting the enemy.
Just FYI: the smile was genuine. Kimblee is a definite sociopath, but he also holds deep admiration and respect for people who stick to their convictions, such as the Rockbells.
A non-villainous example is when Grumman's Xanatos Gambit to prevent Bradley, A.K.A Wrath, from being able to reach Central, when Mustang's assault happens, which either Bradley will not return to Central due to a training exercise or does return and his train is destroyed while over a bridge planted with explosives, possibly killing him, pulls off. Grumman has this reaction when he hears that the later happened. It's even better in the anime where the opening notes of the ending theme play right when he does this.
While almost everyone in Gintama does this from time to time, Okita and Takasugi probably do it the most.
Haruko from FLCL usually has one whenever she is fighting.
Tomomi Saikyo of Ladies Versus Butlers!! is commonly seen with this when she is about to/planning to screw over a classmate of hers. It's hinted that she manipulated Akiharu Hino, a childhood friend, into saying, "I want to become a bride."
Characters driven insane in Soul Eater mainly get full-on Slasher Smiles. It's just the look that's used. In his moment of madness, Death the Kid gets a smirk not entirely unlike his expression when pleased under normal circumstances. Somehow this makes it worse.
Ami, like her canon counterpart, is as about a nice a girl as you can get. But occasionally she'll have a really 'good' science related idea or figure out how to get back at a villain in the perfect way. Then a very disturbing smile appears on her face, and it really doesn't help that her eyes glow red
Emperor Palpatine from Star Wars does this quite a bit, one of the most notable examples would be when he was chuckling to himself behind Vader's back during the latter's infamous Big "NO!".
Sean Miller, an Irish terrorist in Patriot Games, uses this on Jack Ryan during his own trial for murder and attempted kidnapping.
Arnold Vosloo's smirk is probably the main reason he got the part of Imhotep in The Mummy.
Although the fact that he looks rather tasty in a loin cloth probably didn't hurt, either.
The Omen (1976) with Damien. At the end, during Robert and Kate Thorn's funeral, he turns towards the camera and gives a little smile. Damien's nanny, Mrs. Baylock, also pulls off the trope, most notably when she's about to kill Kate.
Most of the head looters in Atlas Shrugged. Especially Cuffy Meigs, Mr. Thompson, Dr. Floyd Ferris, and Balph Eubank. Along with the producers: Francisco D'Anconia, John Galt , and Hank Rearden.
Smerdyakov in The Brothers Karamazov has a habit of flashing these when he thinks he's making an irrefutable point. He can't help himself from grinning sickly here and there when he's giving Ivan a Heroic BSOD during his mind-blowing Motive Rant.
The Kite Runner gives us the following. Bear in mind that the Psychotic Smirker in question has just raped the best friend of the boy he's smiling at.
Assef smiled. It was creepy how genuinely sweet he made it look.
In one of the Discworld books, this editor forgets which, Sybil Vimes catches her husband Sam smiling, and remarks, "I know that look. You're about to do something horrible to somebody, aren't you?"
In the Everworld series, the witch Senna almost never smiles. However, when she's using magic to attack or to control someone, she occasionally makes small smiles to herself. She never shows teeth, but they're so disturbingly described that they would probably qualify as a Slasher Smile if she was more open with them.
When Grand Admiral Thrawn smiles, which happens very rarely, it tends to be this. He's quite self-possessed.
Roose Bolton from Game of Thrones puts on a flat out disturbing one combined with a kubrick stare during The Rains of Castamere when urges Catelyn via his eyes to pull up his sleeve and see he has his chainmail on thus cluing her and the audience in he's betrayed Robb and the Red Wedding is about to begin.
Walder Frey also sports one throughout most of the Red Wedding.
This seems to be Petyr "Littlefinger" Baelish's default expression.
Arya Stark's is developing very nicely.
Benjamin Linus from LOST does this from time to time. The most obvious example would be when he flashes a psychotic smirk in season 4, after having masterfully convinced Sayid to become his personal assassin after they both leave the island.
Star Trek: Enterprise: Stiff-upper-lip tactical officer Malcolm Reed does this when taken over by a non-corporeal alien in The Crossing, and when he's just plain Mirror Universe evil in "In a Mirror, Darkly" (check out the scene where Reed watches his own ship get destroyed).
Within the first ten minutes of the episode "Innocence", the Judge walks up to Angel, and tries to burn him. Since the Judge can only burn someone with humanity in them, it serves as a marker that Angel has lost his soul. He turns to Drusilla, and says "I'm back," with a very muted psychotic smirk.
Angelus also sports a particularly evil smirk when he's watching Buffy and Willow break down upon being told about Jenny's murder. In fact, Angelus is nearly always wearing one.
In Season 8, Buffy gives a particularly frightening one to a vampire as it's doused in fuel, threatening to Kill It with Fire.
Anna Espinosa, a recurring antagonist on Alias, was fond of this trope. Sark, too, if memory serves.
On Merlin we have Morgana. Dear God Morgana. Everytime she does something evil her Psychotic Smirk goes on a rampage and smacks us in the face. Worse? She does it in broad daylight in front of the people she is convincing that she is still good. And they never notice it.
It does finally blow her cover when Gwen notices it.
When Randy Orton latest would viciously stomping the heads of other wrestlers (most of them legends), presumably giving them career ending concussions. Right before Orton does the move, he gets a sinister look in his eyes and gives a smirk.
Albert Wesker of the Resident Evil games loves this, especially in Resident Evil 0 and Code: Veronica. In both games, he's controlling a series of deathtraps that the player has to navigate through. He also smirks evilly when he takes on Code: Veronica'sBig Bad; after finding himself over his head, he smirks and leaves the two of you alone to work out your differences.
Sephiroth from Final Fantasy VII tosses Cloud a cold, calculating smile as he basks in the flames of the protagonist's hometown (although Crisis Core retconned it slightly so it resembled more of a sorrowful or rageful expression).
In Advent Children, Sephiroth's Psychotic Smirk is clearly in evidence during his big fight with Cloud. That, a bit of Dissonant Serenity, and the fact that he never breaks a sweat or breathes heavy during this fight was supposedly done to show his immense strength.
Sephiroth also displays a charmingly evil smirk during his boss battle with Zack in Crisis Core.
At the start of Mass Effect 2, Miranda is seen first conferring with The Illusive Man and then watching over Shepard with a genuinely warm smile. She reappears after losing contact trying to aid Shepard, flashes a smirk at Wilson, before killing him for trying to lead Shepard into a trap and abandon her to die.
A rare heroic example, perhaps, would be Commander Shepard him/herself. While the Renegade Shepard does this quite often, there are two particular occasions when even the Paragon Shepard cannot help but break out that signature Shepard smirk that says "I just saved the galaxy and I'm not even tired. What can you do, suckers?" These occasions are when Shepard emerges quite alive from the rubble after the battle with Saren/Sovereign and when Shepard tells the Illusive Man to go fuck himself right after blowing up the Collector base.
Mass Effect 3: When questioned by Admiral Hackett on how s/he got the krogan and salarians to cooperate (answer: by sabatoging the genophage cure), Shepard's response of "careful diplomacy" is accompanied by a Kubrick Stare and Psychotic Smirk.
Team Fortress 2's Mac Update trailer features a Scout that meets up with his team's Demoman and Heavy (who are enjoying music courtesy earbuds on thin, white wires), trades his clunky 1950s headset for a matching pair of sleek white earbuds, slings his bat over his shoulder, and unleashes the smirk.
Norman Jayden from Heavy Rain sports one of these in the ending "Uploaded" right after scaring the shit out of Carter Blake in ARI.
Alex Mercer does these from time to time. His habit of flashing them at victims when he believes them at his mercy means they border on slasher smiles a lot (and his dietary habits sure don't help dispel that impression). He's just not a naturally reassuring sort of person.
In a flashback, Joshua of The World Ends with You pulls a deeply unsettling one of these right before shooting Neku in the head. He gets another one in the endgame when threatening to do it again, though the smile gets softer after he actually does.
undyingUmbrage of Homestuck was always a bit of a creep. But when he is finally introduced and shown apart from his screen name, he causes a black hole purposefully, and smirks in front of it as his entire universe is destroyed.
Speaking of Critic, he spends most of "The Review Must Go On" like this. His even doing it when he's back at his desk made people theorize that he Came Back Wrong. He does it again at the end of The Cat In The Hat when Soulless is screaming in hell from his torture suggestion.
Megatron and Starscream, in whichever series they feature in, often sport this expression when a plan is going well.
Galvatron can be seen doing this before the ad break (complete with sparks coming from his head) of Five Faces of Darkness Part II and sports this expression in Season 3 whenever he's in one of his more malicious moods (either through formulating a devious plan, just plain ol' torture or both).
Kim Possible does this twice in So the Drama's climactic battle, and again shortly before it. In fact she pulls it a number of times, when confronting Drakken after destroying the Bebes or when throwing around the idea of her playing soccer in "Coach Possible".
Surprisingly, My Little Pony: Friendship Is Magic has Fake!Cadance pulling this off from time to time, though one of her more memorable ones is when she has a sinister smirk while in front of green fire when she sends away Twilight to the crystal caves.