The Ables: Finch seems like a nice old man. By the climax, the mask has completely fallen away, and he shows himself to be the foaming-at-the-mouth psychopath that he is, before trying to nuke Freepoint.
In Across the Universe (Beth Revis), this fits Luther. He hides the joy he takes in inciting unrest among the already unhappy population of Godspeed and love of rape with a pleasant public demeanor. One rather creepy scene has Amy (who Luther had been stalking and trying to rape) bump into him in public. She's in disguise and he doesn't recognize her, so he nicely asks her if she'd like to come see a protest being set up. The instant he figures out who she is, he begins to act terrifyingly predator-like around her.
After Dark, My Sweet: Uncle Bud is a friendly, charismatic old man who is very good at ingratiating himself with people and making himself look like a friend. It's all a facade, and whenever things don't go his way Bud quickly reveals how cruel and short-tempered he really is.
Animorphs: Visser Three likes to present himself as a reasonable, charismatic leader. In truth, he's a complete psychopath, who given the slightest provocation will savagely murder the person he's talking to. As the series goes on, and he loses more and more ground against the Animorphs, his ability to put on a Mask of Sanity steadily degrades.
The Padmasan rulers take this trope and run with it. Although they are Obviously Evil — engaging in war crimes, widely practising slavery and treating common people (especially women) like dirt — they will act polite, pretend to be your friends and offer you riches and power if only they think you can be useful. They don't mean any of it, though, and see you only as a pawn. When Bazil is imprisoned in Tummuz Orgmeen, the Doom ruling the city has him brought before it, then claims he is its friend, offers him wealth and honors and promises to make him a general in its army. Naturally, Bazil tells it to shove off. In book four, when Relkin is captured and interrogated by Heruta himself, the latter tries to sway him to his side by acting friendly towards him and offering to make him king of Marneri. Relkin sees through his lies right away, but decides to pretend he swallowed them.
There are very good reasons why Waakzaam is known (among other things) as the Deceiver. When he thinks that using brute force in conquest of a new world is not a feasible option, he will come to its inhabitants with a wide smile and smooth talk, offering them his friendship and pretending to have the best intentions (usually by promising to make their world great and glorious again). In reality, he sows dissent between them, setting them against each other until they basically destroy themselves. Once they are too weak to defend themselves, he drops the act completely and slaughters the remaining opposition. He already managed to conquer many worlds that way (like the Eleem home world of Orthond, which is visited in The Wizard and the Floating City side story), but when he tries it against Ryetelth and Argonath empire, it fortunately fails.
Thrembode can be charming when he wants to, but it's only superficial. That is how he managed to get into princess Besita's good graces.
Gadjung's really poor at that — though it's probable he doesn't really care. Even after using his magic to manipulate the weather — so that the Argonathi fleet is drawn to his island — and using his magic to mind-control the sailors, he still has the gall to openly lie (via his emissary) that it was Argonathi dragons and soldiers who attacked him unprovoked and that he is actually their friend.
Blood Meridian: Judge Holden is a pleasant and extremely learned man who never fails to entertain his comrades. His comrades are a mercenary army that does ethnic cleansing (specifically of Indians) for hire, and even among this band of murderous psychopaths, the judge's atrocities eclipse them all; he is a mass murderer and a pedophile motivated by nothing but the infliction of pain, and he's probably a Humanoid Abomination to boot.
Jiggs Scully is a friendly, jovial guy, but his casual racism and ruthlessness offsets it. He even casually jokes with George while threatening to kill him.
Andres is quite formal, but it barely masks how much of a vicious killer he is.
The Chronicles of Narnia: The White Witch, Jadis, demonstrates a personality like this in both The Lion, the Witch, and the Wardrobe and The Magician's Nephew. In the former, she ensnares Edmund with food and drink and promises of power. She is far subtler in the latter, however, with words so honeyed you could hardly believe so "fierce" a person had said them. Her affability and feigned concern for Digory's mother nearly succeeds in persuading him to disobey Aslan, but her genuine lack of concern for anyone that isn't useful to her trashes the whole thing.
Alex:[rips a crystallography book out of mug victim's hands] Excellent, really first-class. But what is this here? What is this filthy slovo? I blush to look at this word. You disappoint me, brother, you do really. Victim: But— but— but...
The word in question, by the way, is implied to be “cleavage”.
A Court of Thorns and Roses: Amarantha can be quite charming when she wants to be, with even Feyre noting that if she hadn't known better she'd be taken in by her, and she often comes off as politely-spoken and genial when interacting with Feyre. It's made obvious that she's just toying with her and she feels nothing but malice towards her.
David Copperfield: Uriah Heep may well be literature's most triumphant example of this trope. Even as he openly hints his scheming to the main character, he constantly put on a fawning, "'umble" air.
Greg Stillson in The Dead Zone is great at appearing to be a nice enough, if a bit goofy, guy in public, but he has a very nasty side that he tries to keep under a mask of civility — although he doesn't always succeed.
In The Death Gate Cycle, Sang-drax treats Haplo like an old friend, and it's made clear he does it just to mess with him rather than out of any feelings of genuine affection. He's also shown to be capable of sounding positively cheery when describing the Fate Worse than Deathhis people have planned for the whole universe. Genuinely thwart his plans, though, and he gets very nasty very quickly.
Jack the Ripper in Alex Grecian's The Devil's Workshop, who thinks that he's doing his victims a kind of favor, and sometimes expresses concerns about their comfort or state of mind.
In Carpe Jugulum, the vampire Count de Magpyr actually thinks he's being Affably Evil, or Above Good and Evil, or not evil at all. He's just being "reasonable" while dominating, destroying and enslaving others, and acts with apparent politeness. His mind-controlling powers can make people actually believe he is that nice, even when he's talking about blood-sucking arrangements. His family is similar, except for Lacrimosa, who's a sadistic teenager with no patience for being "nice".
Carcer in Night Watch Discworld is an unrepentant serial killer who will gleefully deny all responsibility for his crimes with a charismatic grin, and then casually stab you between the ribs before you can argue.
Mr. Teatime can be this way. Especially in the Sky OneHogfather TV film.
In Bram's Stoker Dracula and most of its adaptations, the Count gives a polite and inviting first impression, if somewhat unsettling. The guest usually dismisses the last part under cultural/foreign differences.
Dracula: Welcome to my house! Enter freely and of your own will. Go safely and leave something of the happiness you bring.
In Dragon Bones, high king Jakoven. Acts all nice while reminding people that he knows where their family lives, and how terrible it was if anything happened to them.
The Dream-Quest of Unknown Kadath: Nyarlathotep, the crawling chaos, is an Eldritch Abomination, but he welcomes Randolph Carter politely, explains why Carter hasn't been able to reach his marvelous sunset city, and gives him the means to fix the situation. Only later does Carter realize that Nyarlathotep has lied to him and engineered his doom just For the Evulz.
Nicodemus is a 2000-year-old human who made a pact with a Fallen Angel to essentially inflict as much misery and pain on the world as he could; in his own words, he believes in Armageddon as less of an event and more of a mindset, one that he wants to share with the world. He is also incredibly polite and friendly and civil, even while he's got you tied up and offering you a choice between recruiting you for his organization or getting your throat cut. The affable demeanor hides a genocidal monster and doesn't waver even when he's getting ready to unleash a deadly plague across a country or while his minions are kidnapping and torturing a twelve-year-old girl.
Even more so Duke Ortega of the Red Court. Nicodemus is polite whenever he has no reason not to be, generous when it costs him nothing, and appreciates the "artistry" of a classical painter and a serial killer on equal terms. Ortega is a pure barbarian who has spent his unlife carefully learning to pretend to have ethics, manners, and reasons for his actions beyond the basic infliction or avoidance of pain for those times the fiction proves useful.
Lord Raith is polite and well-mannered and can put up a friendly and charming facade when he wants to. In reality though, he's a horrific Abusive Parent who rapes his daughters into submission to break their wills and will callously murder his own sons just to keep his power for himself.
Virtually everyone in the Winter Court who's not too savage to have a conversation with has this going on. They'll be polite and courteous one second and then show they're vicious and hedonistic sociopaths the next.
The titular Doctor in Dr. Franklin's Island is smoothly urbane and smiles a lot. He's never more than mildly exasperated at his Defiant Captives or his colleague with the inconsistent conscience, but said captives are always aware that he doesn't actually think of them as people and will quite happily vivisect them.
Dust Devils: Adam Price can be charming and polite and pretend to be a rather affable actor. That's just a front he puts on, as he's a deranged maniac who enjoys killing and mutilating.
The First Law: Bayaz seems to be a friendly and somewhat curmudgeonly old man with high expectations and a sharp tongue. Ultimately it's revealed that he's a thoroughly evil chessmaster who pursues power at any cost. All of his casual, grandfatherly posturing is just a false veneer over a cruel and merciless will.
In Fool, a Perspective Flip retelling of King Lear, Edmund starts the play as a fairly open Jerkass, but partway through he has an offscreen epiphany that his attempts to be a Manipulative BastardChessmaster would probably be more successful if most people weren't repulsed by him and his company, leading to the page quote where he discusses this trope with the book's protagonist.
Goosebumps: Vanessa from Chicken Chicken is a witch obsessed with manners, and punishes children who show her poor manners, but inflicts horrific Disproportionate Retribution and is needlessly cruel to the children she punishes, outright laughing at and mocking them (despite her obsession with manners and this cruel behavior showing ''her'' as being very rude herself), only restoring them to normal when they first write her an apology note for making her drop her groceries by accident and not apologizing after doing so initially, and then a thank you notefor "teaching them manners". She does offer them sodas after everything is said and done...but than she casts another spell to turn them into pigs just because the boy burped without apologizing and the girl laughed instead of reprimanding him.
Ghost Roads: Bobby Cross is a fairly charming individual who speaks in a fairly calm and polite voice but none of that is genuine. Bobby underneath that is a sadist and sociopath who murders so he can selfishly live forever.
Harahpin gives us Korganest, who is pretty polite when he converses with his victims, which makes it all the more terrifying when he suddenly raises his weapon and strikes.
Voldemort exhibits this trait a few times, mostly in Goblet of Fire when he makes some terrible "hand" puns to Wormtail during a related task. From what the reader sees of his younger days, he was even more of this trope before his sanity was completely gone.
Dolores Umbridge is worse at this than Voldemort, and possibly more hated to boot. She makes Hogwarts into a dictatorship in Harry Potter and the Order of the Phoenix, but her office is decorated with gambolling pictures of kittens, and she wears knitted sweaters and bows in her hair. This is magnified by the fact that while Voldemort does his terrible deeds towards an end with some sort of magnificence, and was prepared to embrace his pure-blood enemies as allies if they turned, Umbridge merely gets off on her ability to make people suffer.
Lucius Malfoy is also an example. His tone is almost always even and coolly polite, but his words tend to range from mildly to extremely insulting to whoever he's talking to.
The Hate U Give: The drug lord King, though at first seeming Affably Evil, is slowly revealed to be this. Sure, he can be polite, amiable, and even charming at times, but as his true colors become apparent, we see that he's rotten to his core. By the story's end, he's attempted to murder a child and all but placed a target on the back of our (sixteen-year-old) protagonist for "snitching" on him. Luckily, he's stopped and gets arrested.
In Exile's Duology (Heralds of Valdemar), Lord Orthallen straddles this, most vividly seen (or not, rather) when he was acting as Selenay's confidant.
President Snow of The Hunger Games. He speaks to Katniss in a polite manner while threatening to have her and her loved ones killed if she makes a mistake.
The In Death series: A number of the murderers in the series are very much this. Sure, they will act like they're so nice and polite, but that is just an act. A number of them are The Sociopath, which might explain it.
Jaine Austen Mysteries: In "Nightmare on Elf Street", Barnaby King seems like a great guy, complimenting Jaine, being great with the kids in Santa Land, and even running a charity that helps less fortunate kids get toys. In reality, it's a scam that's only lining his pockets, and he is more than willing to commit murder to protect that secret.
The Gentleman With the Thistledown Hair, from Jonathan Strange & Mr. Norrell. The scene where he's so happily, enthusiastically telling Steven Black how many people he's had to kill in order to try and restore his true name to him makes him seem like a big, friendly, amoral murder-puppy.
Journey to Chaos: Mr.15 speaks in a friendly and polite tone to everyone, even to captives that he plans to experiment on or people he's moments away from killing.
Kings of the Wyld: Kallorek "the Orc," Saga's old bookie, acts like a friendly and eccentric landowner. But he reacts violently to any insult, real or imagined, casually breaks his word on both major and minor deals, and has a monopoly on all booking west of the Heartwyld that lets him take fully half of anything any mercenary bands make.
Nicolae Carpathia, The Antichrist from Left Behind, initially seems to be the humblest, most empathetic Son of Satan you'd ever meet, and comes across as a Well-Intentioned Extremist at worst. However, this is all entirely an act; in truth he's a completely self-centered sociopath and a two-dimensional thug. The movie version of Nicolae actually appears to be more genuinely Affably Evil, although still a ruthless megalomaniac.
Legacy of the Dragokin: Man in Shadow is casual and affable at all times, even when threatening to rape the person he's talking to. By the way, said person was already a victim of rape and he might have known this.
The Lord of Bembibre: The evil Count of Lemos is unfailingly polite and soft-spoken, even when he is torturing his victims. At one point, Beatriz notes his politeness rings hollow and artificial precisely because it is so meticulously calculated and carefully calibrated.
Shadowthone can be rather friendly, when he's not yelling in exasperation or giving deadpan sarcastic commentary, though the patina is thin and he can go from calm and threatening to shrieking mad in seconds.
Clip tries for this during the events of Reaper's Gale while travelling with Silchas Ruin's group, chatting affably about non-consequential things, affecting politeness and pretending not to notice the stealth insults he throws around. At the same time, Clip does not even try to hide that he's an assassin and drops the act as soon as there's no one he can rile up with it anymore.
The Man with the Terrible Eyes: The Supervisor is very good at affecting a calm, warm persona even when he's about to conduct horrific experiments on the Man while he's awake and conscious and then healing him and erasing his memory so he can keep doing it. Over the course of the story, the Man gradually realizes that it's just an act and that the Supervisor checking up on his health and saying that he thinks of him like a son has absolutely no bearing on what mind-bendingly awful things he'll do to him if given the chance.
Matilda: Agatha Trunchbull. At one point, she gathers the entire school in the gymnasium and orders Bruce to the podium, blaming him that he pilfered the cake that belonged to her. He outs himself by saying that his mother's cake is better, to which she politely replies that he can't know for sure until he tries some more and has him eat a huge piece of the cake, and after he's done, she asks if he liked it. After he admits he did, she brings out the rest of the cake (18 inches in width) and orders him to eat it in front of the entire school, planning to punish him for eating some of her cake by forcing him to eat the rest until he gets extremely sick. When he manages to do so thanks to the cheers of the other children and doesn't get sick at all, she smashes the plate over his head and gives the entire school 5 hours of detention.
The Mental State features a psychopath calling himself Saif Dhu Hadin. Typical of psychopaths, he worms his way into the confidences of others by promising to help them get what they want. Only when they try to leave or think for themselves does he reveal how barbaric his punishments can be. His initial tactic is to amputate his victims' fingers. This is followed by threatening the lives of their families. He even pays for one of his informants' daughter's medical bills just so that he can threaten to cut her off and leave her to die unless his accomplice obeys him.
General Zaroff from The Most Dangerous Game actually starts off as genuinely Affably Evil, but he quickly slides into Faux Affably Evil. When the protagonist Rainsford arrives at his home, he lets him dine with him, complements him on his book, shows respect for a fellow big game hunter, and reveals he has turned to hunting humans. He forces Rainsford to be his next prey, and still acts polite the whole time.
O'Brien in Nineteen Eighty-Four. He talks to Winston in a calm tone while torturing him.
Evening Winterrose serves as a recurring villain in the series and can keep up a decent facade of being polite but it's just an act to hide a petty monster.
Treasa Riordan is friendly and charming but it's clear that it's just a thin veneer that attempts to conceal an ambitious sociopath.
King Rhys can be charming and charismatic but that's just a sham. In truth he's a sadistic creep and deeply prejudiced individual with a distinct Lack of Empathy for others.
Gloriosa, from Our Lady of the Nile, actively uses her image as a harmless, model high-school girl and activist to get what she wants (which usually involves people getting hurt).
In the Redwall series, Ferahgo the Assassin is this. As the narration puts it, he "smiles a lot, but he never jokes".
The Ripple System: Well, calling him "evil" might be a stretch, but Tyrann is a selfish prick who is conquering the entire game world using his army of zealous followers, and making Ned look the like the one with an unfair advantage. He manages to keep up a smile most of the time, but Ned can see under it that he is very annoyed whenever Ned survives and ruins Tyrann's most recent plans.
In River of Teeth, Travers is a sleek and well-spoken man who is always impeccably dressed and exceedingly polite, even when he is cutting someone's ear off or throwing them to the feral hippos for the act of cheating at cards or burning down someone's ranch because they refused his offer. He is also not above threatening someone in public, he will simply be very polite while doing it.
Scary Stories to Tell in the Dark: Samuel Blunt of Wonderful Sausage is described as a "fat and jolly butcher". Said "fat and jolly butcher" kills people and grinds them into sausage to serve to his customers. There is nothing truly jolly about him.
The Screwtape Letters: The demon Screwtape speaks like a supernatural version of an archetypal Obstructive Bureaucrat, always superficially polite, but in the most unpleasant and condescending manner possible. And, like most demons, while his preferred prey is humans, he is openly perfectly happy to eat his colleagues, including his own nephew, should they fail in their tasks.
The Supreme Custodian in Septimus Heap has mannerisms and charisma, but readily throws people he dislikes into jail and is otherwise a pretty nasty person.
The Sisterhood Series by Fern Michaels: Henry "Hank" Jellicoe, despite having elements of Affably Evil, might actually be this. Cross Roads reveals that he is at least a He-Man Woman Hater and at most a Straw Misogynist. He treated his wife Louise like she was just a servant and didn't care if she overheard his conversations on the illegal dealings he made. She managed to escape him, and he has never been able to find her. He very much wants to kill her for having the nerve to leave him years and years ago!
Roose Bolton is always polite and soft-spoken and has a rather amusingly dry sense of humor when in company — to the point that most nobles who don't know him that well assume he's the Affably Evil man he appears to be. Smallfolk learn better very quickly. He's a cold-blooded sadist who can casually threaten to cut off a child servant's tongue for talking too much, who can calmly discuss tortures or rapes he's performed, and who allows his son to rape and torture as much as he wants, as well. Just so long as it doesn't interfere with his plans.
Speaking of Ramsay Bolton... He can really turn the charm on when he wants to, in a rather folksy kind of way. However, unlike his dad, he can't sustain it for more than an evening, tops. His inner monster is all too easily brought to the surface for all to see at even a hint of potential fun.
Littlefinger sits uneasily between this and Affably Evil. He always has a joking, flippant attitude even around people he's about to backstab (which is anybody), and that does seem to be genuine to the extent that he acts the same way around those he (probably) isn't plotting against (at that moment). What makes him more Faux Affably Evil is that he seems to lack genuine emotion and empathy toward others; even when he's actively helping somebody, there's always at least the selfish reason of wanting to have them around for his own sake or for any future benefits that might come his way, rather than even the tiniest concern for their longer-term well-being. As all to often, the "help" he gives is as much of a means to eventually undermine them as it is a ploy to get his claws into them just to use them.
Both Cersei and Joffrey can turn the charm on to be very polite and civil at official functions when they really want to. However, Mother Dearest is rather better at long-terming it than her Bonny Baby Boy is; just see how much longer it took Sansa to work out that she was almost as rotten. Yet, both are cruel, vindictive Brats with impulse control issues and massive, easily bruised egos. He very much learned it from her.
Natali Stretesky of A Specter Tale (and its sequels). She is charming, considerate, a gracious host, and innocuous enough to get normal men to date her. But she will not hesitate to end you if she finds (or invents) some reason to.
During Galaxy of Fear the Big Bad, in his first appearance outside of Villain Opening Scenes, is condescending, looks at an infected girl like she's a piece of meat, and does some quite heartless things. But for the most part he also radiates mocking friendliness, praises the resourcefulness of the heroes—even though that admiration turns to rage whenever they thwart him—and exits the book with an "Excuse me once again."
In Star Wars: Kenobi, Jabba the Hutt and his thugs. Jabba himself is an unseen presence in the story, but he maintains a townhouse in Mos Eisley to give the impression of a benevolent interest in city affairs. His accountant Mosep Binneed meets Orrin there and puts on an air of paternal concern as they go over Orrin's swelling debt and he makes thinly veiled threats of torture and death if it's not paid off.
Tales of Kolmar: Berys clearly enjoys himself when doing various terrible things, and at those times is usually rather cheerful and upbeat.
In The Lord of the Rings, Saruman is shown to be highly friendly and charismatic when speaking to people, to the point that his voice is described as magical. Even after he has killed many of their countrymen, he's able to leave many of the Rohirrim on the fence when they come to parley with him. However, when Théoden bluntly tells Saruman that he deserves death for what he's done, Saruman immediately snaps, and his honeyed words give way to utter contempt, shouting crude insults at Théoden and besmirching the same lands and house he was complimenting just a few moments ago.
Sauron, more in The Silmarillion than in The Lord of the Rings, where he's mostly offscreen. Over and over again; the charming snake in the ear of anyone who listens. This is one of his most defining traits and that's why he is known The Deceiver. That is, until he loses that power or shows his real nature...
Smaug, in The Hobbit. Intelligent, articulate, wickedly charismatic, and a polite conversationalist. He has a fondness for riddle-games and actually enjoys himself during conversation with Bilbo (initially). He's charismatic to the point that Bilbo is sorely tempted to take off the Ring and reveal himself and even manages to make Bilbo doubt the intentions of Thorin. However it's also a very bad idea to piss him off, as Smaug proves downright terrifying when angered.
The Big Bad of the Song of the Lioness quartet, Duke Roger. He plays up the What's Up, King Dude? angle by telling the pages to be informal with him and acts like a Cool Uncle to Jon... while plotting to kill him so Roger is first in line for the throne again. George calls him Alanna and Jon's "smilin' friend" and Alanna is suspicious of him for being too friendly.
Ozorne in Emperor Mage plays nice for much of the book, being very civilized and sometimes personable around the delegation from Tortall. While this is genuine around Daine because she helped his treasured birds, but every other character, even her monkey friend Zek? His true colors are without any warmth at all. Hints of this trope show in the next book, but they stop by the end.
Dolsa from the Provost's Dog series will gladly and civilly explain that she murdered dozens of people, tortured others, and is intent on murdering the king and queen all because the Crown had the gall to tax and license mages. Oh, and she'll brainwash anyone in her vicinity to think she is the most wonderful person ever because she thinks she is.
The Tough Guide to Fantasyland: Bad kings will generally be very polite (until of course they show their true colors on their generous offers to the heroes being refused). In fact, the more polite toward the heroes a king is, the more likely they're bad. The more polite in general the worse kings turn out to be.
The Twilight Saga: Aro, head of the Volturi, puts forth a very charming persona to hide how power-hungry and ruthless he is.
Vanity Fair has this in Sir Pitt Crawley of whom the narrator comments "he was fond of drink, of swearing, of joking with the farmers' daughters: he was never known to give away a shilling or to do a good action, but was of a pleasant, sly, laughing mood, and would cut his joke and drink his glass with a tenant and sell him up the next day; or have his laugh with the poacher he was transporting with equal good humour." Notably, the narrator explicitly comments that if the character was slightly more honest, he would be a very successful scoundrel, but instead, is simply too crooked to prosper.
Wings of Fire has Darkstalker, who can be paternal and considerate towards people...when he's on top. The instant anyone threatens his plans for global conquest or IceWing genocide, he will brainwash them into absolute compliance.
The witches in The Witches. Overlapped with Bitch in Sheep's Clothing, they present themselves in a very ladylike fashion and disguised themselves as presentable in order to lure children. Of course the entirety of the species is hideous from inside out, hates children, and is overall morally repulsive.
High-end villains in the Zones of Thought series tend to be this.
Tomas Nau in A Deepness in the Sky fooled everyone, including remnants of the faction which he backstabbed and almost wiped out in cold blood, for decades with his nice guy act.
