Carnage: Alien symbiotic mass-murderer/serial-killer, real name? Cletus Kasady.
Kaine: Spidey's ill-fated *first* clone (*eyes rolling*) and indirect participant/instigator of the Kraven legacy resurgences.
The Kingpin: more famous as a Daredevil acoutrement, he was Spidey's first and has the last name "Fisk".
Kraven The Hunter: One of the purest examples of this trope, his own real family name is even "Kravinoff" (and his sons and daughters are more than happy to carry on the line...or resurrect it, as the case may be).
Charles Foster Kane: With the manipulation of the public via newspaper yellow journalism, the politically ambitious marriage to his first wife, the forcing of his second wife into an undesired and ill-suited operatic career, or even the abandoning of his first wife for said second wife, he's really the antagonistic villain of his own movie.
Extra villainous toppings for both the naturally Xtreme Kool Letterz of his palatial-but-nearly-deserted estate "Xanadu" (effectively transforming it into a villainous stronghold) AND for one of the film's real-life inspirations, William Randolph Hearst, (whose name, sadly, has no K in it) forbidding any mention of it in any of his publications (a villainous, ego-centric move if there ever was one).
Folklore and Mythology
Koschei the Deathless, from Russian folklore. "Deathless" because he's removed his soul from his body to attain immortality. It's hidden in the eye of a needle, inside a duck egg, inside a duck, inside a fox, inside an iron treasure chest wrapped in the roots of a tree on an island which simply isn't there most of the time. Possibly the original lich.
The goddess Kali from Indian mythology; representing Time, Change and Death, she is an unfortunate and unavoidable, but an ultimately necessary, "evil".
Loki from Norse mythology. His name SOUNDS trickster-y. And if he's feeling helpful, that's great. If he's feeling disgruntled - or if he's crossed his moral even horizon - bad things will ensue. Ragnarok, for instance.
For that matter, how about Ragnarok? You know, the Viking version of the apocalypse?
Sekhmet from Egyptian mythology. She was a goddess depicted with the head of a lion and was so literally bloodthirsty she drank a whole lake of wine just because she mistook the red liquid for blood. And didn't start feeling the effects until after she finished the entire lake. Even Ra was afraid to try to stop her rampaging.
The Lord of the Rings: The Uruk-Hai (and the Orcs from which they were bred, to an intentionally lesser extent).
Robert E. Howard was fond of K Names, as evidenced by King Kull and Solomon Kane.
Shere Khan: bonus points for translating directly as "Tiger Lord" (or "Tiger King", if you want to double-up on the K-sounds).
Kaa: although he is more an accompanying protagonist to Mowgli than in the familiar Disney film (where he's an outright villain), he still has great powers, including a hypnotic dance, and commands respect and prestige from the other characters... not only do Baloo and Bagheera sing a song together with him for Mowgli, but it is actually they who ask him for help in rescuing Mowgli from the Monkeys, and also become hypnotized themselves by him during the task, all of which show how powerful and respected he actually is.
Kirschov Latanya of the Second Sons trilogy, though he is more dumb than evil (He was the last person in the entire universe to work out that Marquel was evil, to start...).
Note also the non-Klingon Kodos the Executioner, a double whammy (although he gets some sympathy in the end of his episode).
Khan, also listed in the film folder.
From the perspective of the bad guys, 'Kirk' is probably a name to run really, really fast from...
As well as Spock; Vulcans are a super-strong-telepathic-able-to-knock-you-out-with-a-pinch race that are not to be trifled with.
"King Kong! King Kong! The White Man done you wrong! And so we sing this song to you! Mighty Kong! King...Kong! Oonga-boonga, hoonga Kong! Get a load of mighty Kong! A million natives can't be wrong! Oonga-boonga, hoonga Kong...": This not-famous-enough vinyl tribute to the film original is also a direct descendant of ooey-gooey gorey #1 classic by Bobby "Boris" Pickett, the "Monster Mash".
Helter Skelter: Infamous due to events beyond its control, this song was originally conceived when "the cute one" read that The Who had produced one of the "loudest, rawest, dirtiest" rock songs... and thought that theycould do better. It starts to veer slightly away from its badass origins, however, once you realize that a "helter skelter" is actually a playground slide and that this song could just as easily have been about "going 'round and 'round on a MERRY-GO-ROOOOUUUNNNDDD!!!!" or "riding up and down on a FERRIS WHEEEEEL!!!!" (although the phrase "helter skelter" itself does have a double-meaning of "going crazy", "chaotic" or "without order").
Nikolai Volkoff & The Iron Sheik: A "Soviet" heel who, at the height of 80's Soviet/American rivalry, had the audacity to sing the Soviet National Anthem before every match, and his authentic Iranian tag-partner who not only had bodyguarded for the Shah, but also competed (for Iran) and coached (for the U.S.) in the Olympics.
Killer Kowalski: Famous 6'4" Polish-Canadian "heel" wrestler (who was actually extremely nice in person), he ripped off an opponent's ear then showed up at the hospital the next day to laugh at his victim about it (in actuality, the two were friends, the incident was an accident, and the two were laughing about the fact that he looked like Humpty Dumpty in his bandages); he was also the first person to *ever* PIN ANDRE THE GIANT!!!
Warhammer 40,000: "Kharn" sounds scary enough, until you find that his title is "'the Betrayer''". You really, really want to run away from this one.
Also, Khorne, Chaos God of War, whose followers are known by their battle cry "Blood for the Blood God!" The 40k universe is not a happy place.
Kaelis Ra, the god of death. Oh, and Kaela Mensha Khaine, the Eldar god of war (as well as murder, violence and destruction, not a really pleasant guy to be around).
In Dungeons & Dragons Orcs and Drow tend to have k and g sounds in there names, so if one runs after hearing a name like Grak or Kagak (which are just random names btw) it's probably best for their health.
Magic: The Gathering has Kozilek, The Butcher of Truth and Emrakul, The Aeons Torn; Eldritch Abominations called the Eldrazi that transcend the colors of mana.
Most of the Always Chaotic Evil species and various fearsome critters in BIONICLE have Ks in their name: Bohrok, Bohrok-Kal, Rahkshi, Vahki, Visorak, Piraka, Skakdi, Barraki, Makuta, Skrall, Kraawa, Kralhi, Kranua, Kraahu, Kavinika, Kinloka, Kahgarak, Parakrekk, Kraatunote (an unreleased promo set), Krana, Kraata, Kuurahknote (and the rest of the named Rahkshi breeds: Turahk, Guurahk, Panrahk, Lerahk and Vorahk), Kohraknote (and the rest of the Bohrok breeds: Tahnok, Gahlok, Pahrak, Lehvak and Nuhvok, plus their respective "-Kal" variants), Keeleraknote (and the rest of the "regular" Visorak breeds: Vohtarak, Boggarak, Roporak, Suukorak and Oohnorak), Klakk... And on an individual basis we have names like Krekka, Krahka, Krika, Karzahni, Krakua, Kopaka, Fenrakk, Irnakk, Kardas, Voporak, Sidorak and Kollorak.
The Ignika, one of the most powerful Kanohi, and the only one that was intentionally made sentient.
Kraahkan, the evil Mask of Shadows worn by Makuta.
King Koopa was probably the first widely-known use of this trope in gaming.
Kyogre: a gigantic, ocean-controlling beast of legend who *wars* with other gigantic, continent-and-air-current-controlling beasts of legends.
Magikarp: raging Gyarados, anyone?
Ekans/Arbok: poison-spewing snakes accustomed to hanging out with the badasses in Team Rocket.
Murkow/Honchkrow: naturally "Dark"-type pokemon who are called "the Summoner(s) of Night".
Skorupi: a poison-type scorpion who evolves into a Dark-type bruiser (the "Dark" type is literally called the "Evil" type in the original Japanese).
Ninjask: a super-speed-demon insect that's so badass, it has a mirror evolution in which the empty *shell* of its former metamorphosis is animated into a near-indestructible phantom (ie: Shedinja).
Darkrai: a legless, hovering Dark-type with the ability to make people have Bad Dreams.
Krokorok/Krookodile: Dark-type crocodiles who live in ancient ruins among a searing hot desert.
Zoroark: A*nother* Dark-type, this one with the ability to create illusions that can fool or even trap others.
Eelektrik/Eelektross: An Electric eel who is immune to his one type's only weakness.
Terrakion: A legendary Fighting/Rock bull who actually made war on humanity with his buddies at one point in the past.
Zekrom/Kyurem/Black Kyurem: As if a gigantic Dragon with an Electric turbine in its tail who is the literal embodiment of the cosmic Yin wasn't enough, he can also be combined with fellow Tao Dragon, Kyurem, to form the stronger-than-both "Black Kyurem".
King K Rool from Donkey Kong Country 1-3 (and Land 1-3, and 64, etc). Speaking of which, pretty much every single Kremling and boss character comes under this: KAOS, Kerozene, King Kut Out, Kleever, Kroctopus, Krow and Kludgel.
Dragon Age: Origins gives us the darkspawn...well, most of them: gemlocks, hurlock, and sharlocks (more commonly called Shrieks in the game, but they're still examples of this)
The Dovahkiin from Skyrim who can evoke this with their real name if the player so desires, also notable are the twin brothers Vilkas and Farkas of the Companions, Kodlak Whitemane, and Ulfric Stormcloak.
Killbane, the big bad from Saints Row: The Third from Act 2 and onwards.
Kilrathi words and names (from Wing Commander) are full to bursting with K's: Kilrathi, Drakhri, Krant, Drakhai, Sorthak, Vaktoth, Hvark'kann, karakh (also Kilrathi for "shit"), Kur Human-Killer, Thrakhath nar Kiranka, Bhurak nar Caxki, etc. Also full of -th endings, R's in awkward places, and loads of guttural spitty sounds (where all those H's come from) for a language that is as Black Speech as you would expect from a race of eight-foot-tall roaring genocidal space lions.
The Legend of Korra has the title character, Makonote incidentally, Mako's namesake voiced Iroh in the first show until his death of esophageal cancer, Tarrlok, the deceased crime boss Yakone (father of Tarrlok and Noatak), and Noatak (better known as the Big Bad Amon). Note that all these characters except Mako are Waterbenders. (Well, Yakone was, until Aang took his bending.)
The prevalence of the hard K in some Russian names may sometimes be used as an intentional Anglo attempt to bring up spectres of Dirty Communists.
Franz Kafka: Author whose works gave his name to an adjective ("kafkaesque") which is "marked by a senseless, disorienting, often menacing complexity" or "marked by surreal distortion and often a sense of impending danger".
American liberal writer/editor Russ Kick
Jacob "Jack" Kevorkian: Do I even really want to go there? All moral ambiguity aside, and whatever view his actions can be seen as, the very word "Kevorkian" can, for some, conjure up images of a bloodthirsty killer who takes pleasure or delight in the calculated terminus of his victims (as opposed to the heroic championing of the right to die with compassionate dignity), and a lot of it has to do with how the media has painted the events...especially in pop culture, where he's always good as the superficial and ill-informed punchline for a quick laugh or two; either way it's probably genuinely safe to say that he might have had an easier ride of it, and a less tough time in the public media eye, if his name didn't have so many damning and damaging hard-sounding 'K's all up and down in it.
The "Mohawk" tribe: their more-familiar name is actually one from their Algonkian enemies meaning "man-eaters"; in their own language, they call themselves Kanienkehaka ("people of the flint") which, oddly for this trope, has even more K's.
Evel Knievel: famous showy daredevil born Robert Craig Knievel, he earned his stage name legitimately from a stint in jail for reckless driving (his cell neighbour was known as "Awful" Knofel, so the guard rhymed him as "Evil" Knievel); although he changed the spelling to specifically escape the "evil" connotations, he's still about as badass as they come.
Ayatollah Ruhollah Khomeini
James Jesus Angleton, Director of the CIA during the early part of the Cold War, became obsessed with the possibility of Soviet moles; intelligence (likely to have been a matter of Soviet disinformation) suggested that one existed, and that his name began with K. CIA agents' careers were blocked and shortened if their names began with K, and the chaos that Angleton's mole-hunt caused seriously impaired CIA morale and function.