This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Smurfs
For the comic books:
The Smurfs had their share of creepy adventures. Like The Black Smurfs, where the Smurfs were being turned into evil black versions of themselves that could say nothing but "G'nap!", and bit each other on the tail to spread the infection.
Worse was the ending of The Black Smurfs. Papa Smurf had been bitten. The only reason anyone was cured was cartoon physics, with the fire spreading the antidote as a gas rather than diluting and burning it on the ground. Otherwise they'd all still be that way.
The worst thing about the Black/Purple Smurfs is unlike the zombies they're loosely based on, they aren't mindless. One of the transformed Smurfs intentionally paints itself blue to make itself look like it's been cured, allowing it to spread the infection in safety. Trying to stop The Virus is bad enough, but having said virus be smart enough that it can counter your lines of defense is so much more chilling.
Oh that is just the tip of the iceberg. The Black/Purple Smurf plague has happened before. Papa Smurf had a book chronicling the disease in exquisite detail, down to a rather vivid color illustration of the source, the "Buzz fly." In fact, the only detail missing was the cure which Papa Smurf discovered completely by accident. There are several chilling implications to this. 1.) The person who wrote that book, witnessing the event first hand, may have had to abandon village with a small group of Smurf survivors, leaving the book as a cautionary tale. Which could mean there's an entire population of "G'naps" out there somewhere. 2.) There are other Buzz flies in existence, which means The Virus can strike again at any time, especially as there's no acquired immunity. (Near the end of the episode, cured Smurfs would be reinfected even as their neighbors were curing others.) 3.) The Viruslearns from its mistakes and mishaps, which means that even if 1.) did not occur because the Smurfs were cured without understanding why (ie, a large batch of that flower pollen just happened to blow through the village for no discernible reason), should the Buzz Fly strike again, the new G'naps may just start seeking out and destroying this flower before moving in on Smurf village. To wit, while Papa Smurf and the Smurfs got more and more desperate, trying all sorts of random things just to see what works, the G'naps were watching, waiting, and planning. They only attacked en masse when word got out that a cure exists, and the G'nap that painted itself blue? In the animated version it was Hefty Smurf, who could wrestle a G'nap to standstill before he got bitten and turned. How tough are the G'naps? It took three Smurfs to subdue Clumsy Smurf while he was tangled in his own net (it was way easier to carry the Smurf from the original version of that scene). Which means that even if Smurfs were able to get close enough to figure out he was just faking being cured, he could easily subdue and turn them.
In the episode Smurf coloured glasses Big Mouth put the glasses on and saw Gargamel and Azrael as food, and it is scary because it gives the audience an idea that Big Mouth is actually going to eat Gargamel and Azreal. (Even worse, he threatened to do that previously, without the glasses.)
Aside from the occasionally creepy "groupthink" vibe common in cartoons during The '80s, The Smurfs had their share of creepy adventures. Like the episode where the Smurfs were being turned into evil purple versions of themselves that could say nothing but "G'nap!", and bit each other on the tail to spread the infection. Or the Christmas Special where they rescue 2 children and Gargamel from an evil wizard, who conjures a wall of fire around himself, the kids, and the Ineffectual Sympathetic Villain, only to fade away screaming in defeated despair under the Smurfs' repeated singing of a sappy holiday song. The ending to the 'G'nap!' episode looked bleak as even Papa Smurf had been bitten. The only reason anyone was cured was cartoon physics, with the fire spreading the antidote as a gas rather than diluting and burning it on the ground. Otherwise they'd all still be that way.
There's also the episode "All Work and No Smurf", in which overworked Smurfs begin to transform into various inanimate objects. Talk about And I Must Scream. They still had mouths and spoke, which makes it worse.
The episode "To Coin A Smurf" has Gargamel actually succeed at turning three Smurfs (Brainy, Grandpa and Sassette) into gold coins. He was happily singing about it as he was headed into town considering how to spend them. Had it not been for him being mugged by a couple of robbers and the three of them being arrested, those three coins could have wound up lost in circulation or a vault somewhere, forever. To highlight how terrifying this is, the coins were fully conscious with the inability to act; they could all still see, hear, talk, and feel pain as several characters bit the coins to test if they were actually gold (which prompted them to go "Ouch" each time).
The episode where Brainy Smurf gets turned into a monstrous Werewolf-Smurf creature from a scratch from a magical plant.
First episode of season four, "Scary smurfs". Monster children abduct few smurfs as part of school project and turn them in monsters. It all turns out well at the end, but it is still very disturbing.
Gargamel's goals were, originally, to either eat the smurfs in order to gain immortality, or use an alchemical formula to turn them all into gold. So, on one hand, he's trying to devour a sapient race for his own selfish goals, and on the other, he's trying to commit wholesale genocide to line his own pockets.
Speaking of eating Smurfs, on several occassions he also threatens and even attempts to eat a Smurf alive, including but not limited to Clumsy, Vanity, Wild, and Baby Smurf. And without the "good always triumphs" standard that keeps him from doing so, he would have a chance at achieving such. Try letting that one sink in for a while.
In "Vanity's Wild Adventure", Gargamel tries to do this twice, Vanity the first and Wild the second time. The first time he tries, he grabs Vanity by his leg and goes straight for eating him on the spot (prompting Scruple to go "Show some class, Gargy!"), and the second time he tries, he says: "When I'm done with this tender tidbit, there won't even be crumbs!" He basically just implied he wasn't going to chew.
The episode "Sir Hefty", involving a huge, Godzilla-esque, green wingless dragon terrorizing the Smurf forest and burning everything it came across with its fire-breath. The sheer destruction the monster causes in its wake is truly chilling, as is the fact the dragon is portrayed as one of the show's most threatening and genuinely dangerous antagonists.
Imagine this from Laconia's perspective in the episode "Smurfily Ever After": you're a mute (and apparently deaf) woodelf girl who's about to get married to a very nice woodelf friend, and everyone in the forest (except for Mother Nature, who sent a calliope as a gift instead telling she couldn't come) is going to attend. All is well and dandy, and the wedding is going perfect. Then all of a sudden, the calliope starts playing terrifying music and ghosts start flying out of it, the scenery quickly shifts from sunny to outright dark, and everyone around you turns into mindless zombies that start walking towards the huge cauldron of boiling water located in the calliope with you unable to snap them out of their trance.
For the 2011 movie:
Seeing Papa Smurf suffer while Gargamel is zapping him is a little unsettling. Smurfette calling his name doesn't help. Also, the scene where Gargamel starts capturing the Smurfs during the climax.
Clumsy learns why he is called thus; shades of Forrest Gump realizing he's just not as smart as most other men.