Soft-Spoken Sadist

Not every villain has to sound like one. Perhaps, instead of having a malevolent rasp, a gloating shrill, or a booming baritone, the villain's voice is instead light and low. They don't yell or intimidate, and might not even be impolite - instead they speak with a soft tone that seems unassuming, meek or even kind, with a soft chuckle and a sort of warm energy... hiding the menace within. The end result is something dissonant and creepy, a monster who might describe just how horribly he's going to mangle you, while speaking in a voice that's anything but monstrous.

See also Affably Evil, Faux Affably Evil, Dissonant Serenity, and Creepy Monotone and compare Wicked Cultured. Note that if and when they eventually drop this soft-spoken demeanor, then that's a surefire sign things are going to get worse. Contrast with Evil Is Hammy. OOC Is Serious Business.

Anime and Manga

 * Frieza from Dragon Ball, in all but the Brazilian dub, where he had a Badass Baritone instead.
 * In the Latin-American dub it's a bit of both: his VA Gerardo Reyero has a deep bass voice, but as Freezer he speaks in a smooth tone and with extremely polite speech patterns. Until he's on the losing side, that is.
 * Sosuke Aizen, Gin Ichimaru, Szayel Aporro Grantz and Shuukuro Tsukishima from Bleach.
 * Digimon Adventure: Of the four Dark Masters, Mugendramon is easily the most prolific killer, effortlessly slaughtering dozens of Numemon with no hesitation whatsoever and being willing to bring down entire cities to capture his targets. He also barely raises his voice above a cold, robotic whisper and has little interest in idle banter. It truly is something to hear him call his Infinity Cannon in a completely lifeless, quiet, deadpan voice.
 * Xellos from Slayers.
 * Yu Yu Hakusho: Itsuki's smooth, mellow voice tends to hide his inner crazy.
 * Kurama is a heroic example. He's always polite and charming, even though he's just as willing to brutally kill or maim (or both) his enemies as Hiei, who generally fights with a sort of controlled anger instead.
 * Light Yagami, who stays cool and calm even while delivering his judgments face-to-face. Poor Naomi Misora.
 * In the American dub of Star Blazers Desslok has an almost feminine voice, which makes him far more creepy than his original bog-standard Japanese villain growl.
 * Johan Liebert, the eponymous Monster. He speaks in the quietest and most innocent sounding voice possible. He's also the one holding the gun to the kid's head at the Complete Monster page.
 * Sailor Moon's male villains tend to speak like this as well. Dirmando, Sapphire and Tiger Eye are some nice examples.
 * Mukuro and Byakuran from Katekyo Hitman Reborn.

Film

 * Kaa from Disney's The Jungle Book (played by Sterling Holloway), especially in the first movie where, despite having the gentle voice of an genial old man, is menacing enough to scare Bagheera. Later, he uses that soft quality in his voice to "convince" Mowgli to trust him.
 * Even creepier when you finally realize where else you heard that voice...
 * The Silence of the Lambs: Hannibal Lecter.
 * Inglourious Basterds: Col. Hans Landa, sometimes.
 * Mr. Blonde from Reservoir Dogs is a very cool-headed, soft-spoken individual who always exudes a casual aura. This makes it all the more unnerving when he slaughters an entire store's worth of people after the alarm goes off, or when he calmly describes to a captured cop how much he will enjoy giving him a slow, painful death.
 * Quentin Tarantino apparently likes this trope. His character Richie in From Dusk till Dawn is a sadistic rapist with a soft, calm voice.
 * Anton Chigurh from No Country for Old Men embodies this trope to a disturbing degree.
 * Gaear Grimsrud from Fargo, when he is not snapping randomly.
 * The Thief of Bagdad has Jaffar, an Evil Vizier and Evil Sorcerer who raises his voice once, in order to cast a spell at sea. It sort of gets rid of the common "who would trust that guy" Fridge Logic about more Obviously Evil viziers: he sounds extremely trustworthy. Too bad he isn't.
 * Toht (the bespectacled Nazi) from Raiders of the Lost Ark.
 * Christopher Guest's portrayal of Count Rugen in The Princess Bride.
 * Jonathan Crane is easily the most softspoken character in Batman Begins.
 * Mister Teatime in the adaptation of Hogfather.
 * Loki in Thor and The Avengers, when he isn't being an zealous would-be ruler, is overwhelmingly cool and polite. When the Avengers are all gathered on the, the   Loki actually seems to be the calmest of them all.

Literature

 * Roose Bolton from A Song of Ice and Fire is mild mannered, courteous, and speaks so softly that others have to listen closely to hear anything he says. House Bolton also has a flayed man as their sigil, and has a legendary reputation for torture.
 * Snape's outward Sadist Teacher personality is described as managing to keep the whole class at attention without going much louder than a whisper in the first book, and even though his speech often contains contempt and sardonicism, he rarely ever does anything but speak softly - which serves to make the times he does outwardly show his rage all the more significant. Also, Voldemort who speaks in an almost snake-like whisper, especially shown during the first chapter of Book 7. He rarely raises his voice, and when he does, it is suppossedly extremely terrifying.
 * Nurse Ratched in One Flew Over the Cuckoo's Nest.
 * In Vernor Vinge's Fire upon the Deep, Flenser is described as having a voice like this.
 * Lord Vetinari of Discworld, in his scarier moments, though he's at least nominally a good guy.

Live Action TV

 * The Reaper from Criminal Minds. He's condescendingly soothing while stabbing Hotch in 'Faceless, Nameless'. And in '100', he uses such a cool, conversational tone to taunt Hotch on speakerphone while calmly playing toy soldiers with Hotch's son. He even covers the kid's ears and spells out D-I-E-S. He plays it so well, the kid isn't even sure if he's a bad guy. This is seconds before he.
 * In The Twilight Zone episode "What's in the Box", Sterling Holloway plays a mysterious but malevolent television repairman who he acts the role of the friendly old blue-collar worker, who gives an exasperated man some extra perks in his cable, and then gets to watch, smile, and subtly gloat as his "product" ruins two lives, never dropping the benign facade.
 * In The X-Files, the Cigarette-Smoking Man has a pleasant, avuncular New England accent. (Except for the times he has a pleasant, avuncular Canadian accent.)
 * Most villains from The X-Files, but in particular the death fetishist who kidnapped Agent Scully in season 2.
 * In the Star Trek TOS episode "Wolf in the Fold," John Fiedler played a serial-killing alien, and he spoke in the same tremorous, timid voice he used when he was playing Piglet.
 * Adelai Niska, Firefly's resident psychotic crimelord, can be very soft-spoken in the tone of a kindly old man. The fact that he does this while he's zapping the living daylights out of you, cutting off your ear, or even worse things makes him utterly rutting creepy.
 * Marlo Stanfield from The Wire. He's very soft spoken and almost never raises his voice -- even when ordering the murder of a man and his family.
 * In Supernatural, Lucifer is very calm, collected, and acts in a casual and gentle manner. He also tortures and kills anyone to get what he wants.
 * The Goa'uld Tanith in Stargate SG-1. Unlike most of his kind, who are very much of the Large Ham variety, Tanith preferred to coldly taunt his enemies before utterly destroying them. Politely wiped out an entire civilization.
 * Wynn Duffy from Justified is a ruthless Dixie Mafia henchman, but he has a very soft, almost meek voice.

Theater

 * From the Harold Pinter play One For The Road, Nicholas is an extremely genial person who happens to be in charge of torture for his government.
 * CLAUDIUS. Dear God, Claudius.

Video Games

 * Yuri from Command and Conquer Red Alert 2.
 * Leon in Star Fox 64 is implied to be a sadist.
 * Dutch Van Der Linde of Red Dead Redemption has a rather quiet, soft voice, which contrasts sharply with his violently insane nature.
 * Ingun Black-Briar in The Elder Scrolls V: Skyrim fits this trope quite well. She's always very polite, happily greets you and thanks you for your hard work on her behalf and rewards you nicely. However, she also reveals herself to be a Nightmare Fetishist with a penchant for alchemy and mixing poisons, largely because she finds glee in the horrors these poisons can inflict on a person.
 * Lord Ghirahim in The Legend of Zelda: Skyward Sword. He whispers to Link that he will beat him within an inch of his life and talks rather casually about burning him to a crisp in the Earth Temple.
 * Bioshock: Subverted with, who spends a good two-thirds sounding friendly, jovial and humane before The Reveal gives us his actual voice. Sofia Lamb plays it straight (ala her Tautological Templar personality). Both were designed to contrast Andrew Ryan's Large Ham persona.

Western Animation
"Pythor: Humbly, I am Pythor P. Chumsworth."
 * Catbert: Evil Director of Human Resources is portrayed this way in the Dilbert animated series.
 * Discord from My Little Pony Friendship Is Magic. He causes madness and suffering, corrupting ponies for kicks and tearing the fabric of existence asunder. All the while, however, he maintains a pleasant and jovial tone
 * Pythor in Ninjago certainly counts. Until he gets angry at least.