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?
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.
Examples
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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 Inu Yasha 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.
Piedmon from Digimon Adventure does this every time he's preparing his next move.
Ken gets a rather Narm-filled scene where he kicks a puppy and then pulls this face. What a douche.
Light from Death Note. Three words: "Exactly as planned."
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.
And now, Sasuke sports one of these in 476, after crushing Danzo (but not really) for speaking ill of Itachi.
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.
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).
Cell's final form on Dragon Ball Z seemed to do this all the time. There's one particular example on the cover of DragonballZ Budokai◊.
Gohan counts too (Super Saiyan 2 Gohan that is)
Aion from Chrono Crusade is very fond of this trope. So much so that it's when he stops smirking that you really have to be worried.
Made all the more frightening in the first season due to Art Evolution just starting...
Ukyo from Samurai Seven is fond of this. And it is creepy.
In Fullmetal Alchemist, Edward, Envy and Kimblee all do this at least once (but then again, we don't think we've ever seen Kimblee when he's not doing this).
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.
Mad ScientistBig Bad Jail Scaglietti of Magical Girl Lyrical Nanoha wears one of these on his face most of the time. He uses a particularly nice one when Due assassinates the founders of the Space-Time Administration Bureau.
The Trigger Happy Joichiro Nishi from Gantz wears one of these at all times, right up until his incredibly painful death. Though of course, he was Dead To Begin With...
Occasionally Eva slips into this when things are going especially well for her. For example, right before having five of her ribs broken... putting the expression under this trope.
Alejandro Corner is fond of doing this as his plans unfold in Gundam 00. And Ribbons starts copying it as soon as it becomes clear that he was just using Corner.
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.
Orga Sabnak, Cortho Bauer, and Sting Oakley of Gundam SEED and Gundam SEED Destiny typically sport one of these. Both Cortho and Sting graduate to full on Slasher Smiles following the deaths of their teammates.
In Kyon: Big Damn Hero, all the Yakuza can do it, mainly because it implies that they can give you hell. Mori's smirk, on the other hand, can cause fear-induced paralysis, and she isn't even one.
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
Film
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 version) 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.
Literature
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.
This seems to be Draco Malfoy's default facial expression.
Chloe gets a small one when Lionel convinces her to do a temporary Face Heel Turn.
Sylar from Heroes does this occasionally. Apparently, Sendhil Ramamurthy who plays Mohinder◊ does a pretty good one, too!
One particularly memorable smirk came while he was shapeshifted as Micah to help him fake his death. For a split second, "Sylar" shows through. The kid nails it.
Lila from Dexter does this in a Revealing Hug with Dexter, having successfully gotten him to come to see her after he'd previously ignored her phone calls.
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).
Male Yandere Masato Kusaka from Kamen Rider Faiz used to do this when he was trying to separate Takumi from his friends... until episode 30, when it morphed into a creepy Slasher Smile.
This seems to be the default expression of Ryan O'Reily in OZ, but seeing as he spends the entire series manipulating one side against the other it's hardly surprising.
Within the first ten minutes of the Buffy the Vampire Slayer 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.
The Jacobi and Simm◊ Masters enjoy doing this too, which is incredibly startling to see on the Jacobi version after he's played a kindly old man most of the episode.
Midnight has such a smirk on the face of whatever's possessing Sky whenever things are going well for it.
The Doctor gets a rather horrifying one when he completely loses it in The Waters of Mars. Behold.◊
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.
The Trickster from Supernatural wears this a couple of times - there's a particularly good example in the episode Changing Channels◊.
From [1], this seems to be Petyr "Littlefinger" Baelish's default expression.
And in Season Three, Roose Bolton sports one throughout most of the Red Wedding.
Professional Wrestling
In Professional Wrestling, Randy Orton's latest gimmick involves him 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.
Tsukihime: Nanaya Shiki. Usually more of an antihero than a villain, his smirk◊ doesn't mean he knows more than he's letting on...it's just that he's going to love kicking/killing your ass.
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.
Sephiroth is pretty much the king of this trope, and Rosso the Crimson is his queen.
Kain smiles for the first time in centuries in Legacy of Kain: Defiance, just before trumping Moebius for the last time. Or, as one Fauxtivational Poster put it, "Kain is smiling. Run for your lives."
Ishida Mitsunari from Sengoku Basara 3 does this in the opening amv after dealing swift death to a couple of mooks. He has a pretty mouth....
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.
Larxene pulls one of these before kicking Sora to the ground.
Since Shadow the Hedgehog is supposed to be a darker version of Sonic the Hedgehog (Stalwart of the Sideways Smile), when he isn't scowling, he will occasionally drop one of these.
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 Wand of Gamelon, Zelda gives a creepy little smirk after she kills hectan.
In "Phoenix Wright: Ace Attorney", Prosecutor Miles Edgeworth does this in court whenever he's about to say something that will crush Pheonix's argument.
Jack—poor crazy, creepy Jack—of Gunnerkrigg Court is starting to make a habit of cracking these. One or two of them have bordered on Slasher Smile.
He got better.
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.
Han Solo in thisDarths & Droids comic, right after Luke questions his sanity.
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.
The picture that used to be above comes from the finale, while Azula is suffering from her Villainous Breakdown. As time goes on, it degenerates into a full-on Slasher Smile.
Psychotic smirks must run in the family, as Ozai is also a master at this. He displays several throughout the Day of Black Sun and Sozin's Comet.
Amon from The Legend Of Korra wears a mask with a built-in evil smirk that's both deceptively inviting and unnerving.
Danny Phantom has Big Bad Vlad Masters who cracks one of these every so often throughout the series, often in front of the main hero as much as he does in secret.
Fairly OddParents: Timmy gets one that takes up most of his face in "Timmy's 2-D House of Horror", when he notices the wave of lava headed towards Vicky's house.
Not to be forgotten, Megatron and Starscream, in whichever series they feature in, often sport this expression when a plan is going well.
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".
Matrix from ReBoot tends to do this, usually right before he's about to start deleting things. This conversation pretty much sums it up:
Ray Tracer: Nice chap. Does he ever smile?
AndrAIa: If he does, run. Very fast.
Megabyte also smirks a lot, most terrifyingly when he banishes Bob into the Net.
It was also one of Hexadecimal's favorite masks, though she seemed to sport it even MORE often after Bob "fixed her face" and let her have actual expressions.
June on KaBlam! does this a lot. Unlike others with this trope, she's one of the protagonists (even if her ideas of fun include putting her friend, Henry, through hell).
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.
Quite a few of Vladimir Putin's "happy" expressions" go into this territory, in the few times his face shifts out of that menacing frown. Transition from Badass Prime Minister to "Are you sure this guy isn't a Bond villain?".
Any newscast about the January 8, 2011 shooting in Tucson will inevitably show this mugshot◊ of the assassin, Jared Lee Loughner. May overlap with Slasher Smile.
