This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Captain Planet and the Planeteers
Sometimes, Gaia shows the potential effects of what an Eco-Villain is doing to the environment on her Crystal Computer to the Planeteers. When the topics of nuclear meltdowns, war, and animal experimentation are shown, the Planeteers get to see some pretty crude things. Episodes like "Planeteers Under Glass", "A Good Bomb Is Hard to Find", and "101 Mutations" deal with such issues. The episode "Old Ma River" also had a short but still unsettling scene where Wheeler and Lita explore a dark, run-down sewage treatment plant.
Some episodes dealt with poachers. The writers did not beat around the bush about it. The Planeteers come across the mutilated carcass of a panther that was skinned and its body left to rot, and one girl is shown a box containing severed gorilla hands to be used as an ash tray.
Most episodes with Verminous Skumm, whether he was trying to conquer cities with his rat army, or giving out drugs that warp people's minds causing them to go insane, and end up with fatal consequences.
Mind Pollution: One of Skumm's scariest episodes has Skumm corrupting the youth of Washington by selling them his latest concoction; a designer drug called "Bliss" that turns the user into a red-eyed, giggling zombie who feels nothing but joy, even as they badly injure themselves. As for what's scary about it...
Boris begging Skumm for more Bliss, in visible agony from withdrawal as he writhes and grovels on the filthy pavement, all whilst Skumm sneers at him and mocks his suffering.
Boris deciding to get Linka to stop bugging him about his Bliss addiction by getting her hooked on the drug herself. It's chilling to think that a person's mind can be so polluted by drugs that they will poison others with the same substance.
The scene where Skumm whips up his withdrawal & Bliss-addled victims into going after the Planeteers. It's like something out of a zombie movie; Wheeler even tries to ward them off with blasts from his ring, but is forced to stop because they just keep coming, and his only alternative is to burn them to death.
The capper? Boris throws himself through a window to get at his cousin, cutting himself badly, then ends up dying from a combination of bloodloss and overdosing on Bliss.
Deadly Ransom: Captain Planet is captured by Blight and Nukem, who shoot down the Geo-cruiser right in front of him. Granted, we find out the Planeteers weren't inside it, but what thoughts must have crossed Captain Planet's mind? Considering how little he reacted to Blight's plan of dismembering him, he probably assumed the worst and didn't care what happened to him after.
It's almost canon confirmation of the Fanon idea that Captain Planet looks at the Planeteers (several of whom are from broken homes; special mention going to Wheeler's father here) as his children; ask any parent their reaction in that situation and few will deny that they'd be as devastated as Captain Planet was.
Two Futures, Part 2: In the dystopian future, after failing to get the other Planeteers to help him, Wheeler decides to visit Hope Island and talk to Gaia. However, he first sees the island as a sleazy pleasure resort akin to Las Vegas or Atlantic City, and then finds Gaia's dead body surrounded by toxic waste.
Summit To Save Earth, Part 2:
Zarm shows the Planeteers a smog covered Paris and some birds trying to escape. One unfortunate bird instantly dissolves into bones.
Talkin' Trash: Wheeler returns home in New York... and suffice to say that episode is just one Adult Fear after another. Culminating with Trish hanging from a helicopter, and one of Wheeler's best friends is killed after jumping on a train.
Planeteers Under Glass: Dr. Blight gets her hands on a virtual reality simulator built by another scientist to experiment on the variables pertaining to the environment's growth and destruction. Blight traps the Planeteers and the scientist in the simulator and goes completely to town on the virtual environment, before all her mucking around results in a nightmarish wasteland that eventually destroys itself. However, amid the destruction, from the ashes of a toppled, polluting citadel, rises some kind of... thing. A cybernetic mutant abomination, human on one side, twisted machinery on the other, with demonic mechanical wings, born from all the polluted, poisonous terain. Silent save for the desperate mechanical whistling of rebreathers audibly failing to coax oxygen from the toxic air around it, the creature falls to its knees, then lifts its remaining fleshy arm to the heavens in a silent Skyward Scream of protest...before it turns to stone and dies, then crumbles into pieces. This abomination has no foreshadowing and just appears, almost as if it's an angel of death meant to bring about the simulation's destruction before dying itself.
Perhaps the worst thing about the cyber-mutate? It's not just the fact that it's recognizably still human despite its cybernetics, in a situation that had until then been filled with just bizarre, alien-looking animals. It's the fact that if you look very close... it's bristling with weapons, including a draping of bullets and a missile ready to fire. This isn't just the last human alive... it's giving us some all too terrifying glimpses as to just what had happened to humanity as it struggled to survive in this toxic hell.
And that isn't even the worst part of it. Crossing over with Fridge Horror, this episode shows that with absolutely NO desire for money or power, Blight is readily willing and capable of brutally murdering and torturing every living thing on Earth, and then casually preparing to destroy even more.
Who's Running the Show?: Blight, Greedly, Nukem and Skumm take over a TV station in order to broadcast eco-unfriendly programming. One of their children's cartoons has a dog's skull being opened up, and having radiator fluid being poured into it!