The Puchuus in Excel Saga. They resemble adorable teddy bears, but plan to take over Earth by any means necessary, even if it means blowing up major population centers. They also have a piss-uglyGame Face when struck and/or in pain, which happen to look like Golgo 13's.
Kamineko, a.k.a. the Biting Cat, deserves mention. While he hasn't killed anyone, the façade of a cute, irresistible little gray kitty that poor Sakaki can't resist petting hides unearthly speed, a rotten attitude, and a set of teeth that resemble nothing so much as a bear trap. His life's mission seems to be to continually tease and beat up poor Sakaki. Most cats seem to attack her out of fright, but Kamineko tends to seek her out with malicious intent.
Maya could also be seen as an example. The cutest ever kitten to come out of a species of wildcat that can kill wild boars. Not only survives a trip from a western island to the mainland, but emits a big enoughBattle Aura to scare off Kamineko AND HIS POSSE. Then he collapses.
Elfen Lied could be described as a manga series about a race of Killer Rabbits. The Diclonius species look like cute girls with pink hair and a slight skull deformation in the form of two horns that appear like cat ears. They are in fact the most dangerous creatures on Earth, with enormous telekinetic powers and the natural instinct to Kill All Humans. The series begins with one of the most powerful of their kind breaking out of a high-security research facility stark naked. She slaughters about 60 guards with automatic weapons and survives a round from an anti-tank rifle to the head.
Kirara from InuYasha is usually a small and cute kitty, but can transform into a giant, flying, saber-toothed, flaming version of itself at will.
The first episode of Slayers REVOLUTION introduced Pokota, a small creature entirely suitable as the possible Non-Human Sidekick... until he was upgraded (almost immediately) to The Rival status with the town-destroying Dragon Slave spell in its arsenal. It's later revealed that his true form is a cute prince of a lost kingdom.
And let's not forget that the first season gave us Tiiba, who just looked like a chicken but was, in reality, a massively powerful monsteronce he put on his mask.
Ururu, despite being extremely timid and withdrawn, possesses otherworldly strength that is revealed when she detects a threat. She goes into a sort of "berserker" mode, and will not stop until either her opponent drops dead or she is immobilized. A prime example of this is her ambush and beating of Ilfort Granz, who manages to wound her in his release form.
Actually, her power has been revealed. She has a "triple sword," which is to say whenever she swings her (appropriately sized, so very small) sword, two giant swords swing along with it. There's also evidently some kind of supernatural aspect to her blades, as she was able to hit an enemy hiding in a pocket dimension with at least one of those swords. And most terrifyingly of all, the two swords that strike with her aren't just floating. They're wielded by two twenty-foot tall monsters who perfectly mimic her movements.
Kon appears to be a useless, perverted stuffed teddy bear making it easy to forget that he was actually created to fight Hollows. His revealed powers include being capable of scaling skyscrapers and being able to jump incredible distances. He doesn't tire, and his kicks are powerful enough to crack open the skulls of Hollows, something only special magical powers and weapons are able to do.
Hiyokokko from Tokimeki Memorial ~ Only Love definitely qualifies, though she doesn't actually kill anyone.
Satchii from Dennou Coil looks like a giant, squishy beanbag chair with a big smiley face that announces its own arrival with a bright, "Me Satchii!" every time. It also fires laser beams that kills kids' digital pets and breaks expensive cybernetic glasses.
Mappy from Dragon Half. Looks like a cat-sized ball of mousy fluff, but can spontaneously transform into a huge, bear-like hulk.
Hayate the Combat Butler: Hayate buys Maria-san a cute little bunny plushy as a present. She then sees the bunny in its TV show, where it declares, "I am Muffy! An assassin from hell! OK! Today, we'll happily cause chaos!"
Kyo Kara Maoh's tiny, yellow, and bred-by-humans Bearbees (which are actually mud with magic on it) will turn into fierce, dark colored beasts that try to kill you if you get them wet. If they're not wet, hey, they're freaking adorable and cuddly. (Those are magical counterfeit bearbees. Real bearbees are cute, huggable, completely non-dangerous, endangered, and their dung makes high-quality paint.)
Mugetsu from Xxx HO Li C is an adorable tube fox who takes to Watanuki and can be seen glomping him, slithering through his clothing, or hanging around his neck — until Watanuki is in danger, at which point he transforms into a normal Kitsune who can detect evil spirits and produce fire.
For a literal example, in Digimon Tamers Terriermon transforms into a trigger-happy rabbit. Correction; he gets guns, he's already trigger-happy.
And of course, even earlier in Digimon Adventure, Patamon digivolve into... Angemon! Heck, forget the digivolving, his In-Training form Tokomon is the Digimon version of the trope namer. Just look at those teeth!
Guilmon is usually adorable and childish, but when in a fight he transforms into a feral beast.
Kirby Right Back at Ya! features the Devil (or Demon) Frog. Not quite a killer, but instead a mind possessor who infects the victim and forces it to go on a rampage (also increasing its physical capacities by quite a bit). An infected Kirby, due to his normal innocence, merely becomes a mean trickster bent on breaking everything and anything he sees. A (voluntarily) infected Dedede, on the other hand, becomes even more evil than normally, and comes dangerously close to killing Kirby.
Pet Shop of Horrors features another literal example in its first episode "Daughter", which involves a rich couple that recently lost their daughter Alice paying a visit to Count D's eponymous shop. Count D sells them a very rare species of rabbit that looks just like Alice. Things go well for the couple until the mother concedes to the rabbit-Alice's request for sugar instead of the water and vegetables indicated in the rules of the contract. It goes very badly for the couple VERY quickly. It turns out that feeding sweets to this rabbit will cause it to "give birth" to (read: get eaten from within by) dozens of flesh-eating killer bunnies that go forth, kill, eat, and "give birth" to more killer bunnies until the town is overrun.
The Teez in D.Gray-Man are golems that look like pretty purple butterflies. And then they eat you. Or Tyki places them inside you so that you can act as a nursery for them to grow and breed in and THEN they eat you from the inside out.
Mobile Suit Victory Gundam: Haros. Those adorable little balls couldn't possibly harm anyone, right? Well, unless you try to mess with Uso Evin. His haro will uppercut you, if you hadn't already frozen up due the machine gun noises it made. If that isn't enough it will use the Gundam, by itself, to destroy you.
A very early episode features adorable "chanela," rabbit-like long-haired creatures that are pastel-colored, smell wonderful, and are adorable as they come. Oh, and they're evil little monsters; THEY GOT FANGS!
There's also a couple killer cats. The first is Rhett Butler, a fat and lazy tomcat who also happens being the reincarnation of the STRONGEST YOUMA EVER (thankfully, he's not hostile). He managed to scare away a whole gang full of feral cats (including one with More Teeth than the Osmond Family). The second, more prominent one, is Artemis: while he's usually on the sidelines, on one occasion he had to fight a Monster of the Week that had just steamrolled Sailor Moon, and actually held his ground long enough for Sailor Moon return with a Mid-Season Upgrade.
Pokémon. Most Pokémon that qualify as cute will qualify for this.
Arguably, none of them hold a candle to this little bundle of doom, though. Pikachu in-game isn't particularly powerful, relatively, despite its billing. But the one in the anime has become seriously powerful. Ash even warns people that his Pikachu is stronger than your average 'Chu. Don't be fooled by the colorful cheeks, the beady, curious eyes; the cheerful smile, the baby-like chubbiness, the way it scampers about on two/four legs, or the adorable way its ears wilt to the side when it's confused. This thing - one (1) of them - has demolished most countries' national defense budgets in buildings, delicate equipment, and Humongous Mechas, struck down two minor gods (one on live television), and regularly packs enough explosive force in a single attack (that by all physical constraints shouldn't even have explosive force) to level forests and send several human bodies flying through the air for miles. Notice that we didn't say "can" or "could". We said "has" and "did".
Chimchar. It looks like a cute, playful little fire-monkey at first, but when the heat of battle activates Blaze, RUN FOR COVER!!
Still worth mentioning is the Togepi from DP142, "Where No Togepi Has Gone Before." It's very, very mischievous and knows friggin' Flamethrower, Extrasensory, Attract, and Substitute.
Dawn's Buneary is a literal example. Granted, she's more of a lover than a fighter, but she can still kick some massive amounts of ass.
In Pokémon Special, the most Badass Pokémon on Gold's team happens to be his Togepi, always doing something Crazy Awesome whenever it fights. It actually took down two pseudo-legendaries!
Bianca's Oshawott gets thrashed by a particularly evil-looking Audino.
Another prime example is Mew◊. The adorable pink floating kitten that can pretty much learn 99% of ALL Pokémon moves ever created. Do NOT be fooled by the lovable appearance—it is extremely cunning as well as powerful. If you piss it off, it will thrash you. Hard. And then it will float away to go play on a windmill. Hilariously, Mew's voice gets surprisingly much deeper when it was focusing its power in the Lucario movie.
In Murder Princess, Ana and Yuni are two unbelievably cute little girls with wacky Anime Hair and sweet little giggles. They're also androids equipped from head to toe with a seemingly endless array of high-tech weaponry (while almost everyone else in the setting is wielding swords). And Ana is just plain bloodthirsty.
The title character in Kimba the White Lion fits this trope to a T. He may be a wide-eyed lion cub, but the fact that he takes on multiple foes several times his size at once, including a group of rhinos, would make you grateful that he's a Martial Pacifist.
Luki and Noki from Dogs: Bullets & Carnage. They're two adorable twin girls... but heaven help you if you agree to "play" with them....
The whole premise of Mao-chan was that an alien race was trying to take over Japan, but were simply so adorably cute in physical appearance that no adult wanted to fight them. So the military heads had to employ their granddaughters (including the title character) as the front line in the war so they could fight cute with cute.
The mad parade in Paprika, a collection of cartoon animals, inanimate objects, and cultural icons that looks incredibly silly and cute. It also happens to be a living Shared Dream born from the minds of a meglomaniac and the insane, and it has the power to drive people to cheerful madness almost instantly-adding them to it's march.
The Dark Mage Zeref. He looks so young and is unbelievably adorable, especially when he cries, and despite not showing up until chapter 200, he's cried on screen more most of the main characters too. You can also die just by getting too close to him, and that's when he isn't trying to kill you.
Let's not forget Pantherlily. When not in his "cute" cat form, he transforms into a human-sized Panther with a Badass BFS
Dragon Ball Z has Gohan, who is normally a sweet little kid who enjoys school. But if you try to kill his loved ones, he will go crazy, transform into a Super Saiyan 2, and slaughter you. It will be quick, it will be painful, and it will be scary.
Fresh Pretty Cure! movie: Hidden underneath the massive armor of the Big Bad ToyMajin and the gathered repressed anger of abandoned toys is... a cute, stuffed teddy bear who just wants to be loved. Love makes sure he gets that wish in the end.
Maetel from Galaxy Express 999. She's a beautiful and gentle woman, everyone who knows her says she's much nicer and defenceless than her sister Emeraldas, is a crack shot, her handgun has enough firepower to take down a tank (it's supposedly one of Tochirou's legendary Hand Cannons, the same rare gun Harlock and Emeraldas carry), and has blown up at least one planet whose inhabitants had enraged her too much. Not to mention she gets awfully scary (as in enough to make a robot hesitate) when she turns serious. Turns out her harmlessness is relative to the most dangerous woman in space...
Honey from Ouran High School Host Club could be the epitome of this trope. He loves cute things, carries around a pink toy rabbit called "Cute Bunny" ("Usa-Chan") and is usually the sweetest kid around, but he's also a martial arts master. Do not make him angry!
Nui Harime from Kill la Kill is a walking definition of cuteness as with her attitude.... who is a total death machine to Isshin Matoi, who she killed long time ago, Sanageyama Uzu, whom she defeated, and to Ryuko Matoi.
KuruKurus in World Conquest Zvezda Plot are a bit of a variation. Alone, they're harmless and cute. In large groups, they're... still harmless and cute, except they can, in some circumstances, merge together into a giant monster. "Killing" the monster transforms it back into a swarm of harmless blob critters.
Jojo's Bizarre Adventure, Part 2: After becoming omnipotent thanks to the Red Stone Of Aja, Cars demonstrates his new powers by turning his hand into a fluffy squirrel... which proceeds to brutally devour one of its own kin and several German soldiers.
Eto from Tokyo Ghoul is a child-like Ghoul that skips around, wears a pink floral scarf, and generally just follows her far more intimidating companions around like a Tag Along Kid. She seems like the least dangerous member of Aogiri and doesn't garner much attention from the heroes, but she turns out to simply being biding her time. She's actually the One-Eyed Owl, the most deadly Ghoul on record and Aogiri's true leader.