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.
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 rises some kind of... thing which appears to be a cybernetic mutant abomination born from all the polluted, poisonous atmosphere which silently turns to stone and dies. The abomination had 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.
And that isn't all that occurs within that part of the episode. The cyber abomination is holding in its arms the either dead or dying body of what is most likely its mate with tears in its eyes. Meaning that Blight just killed the last 2 living humans on Earth. This can also cross with Fridge Horror when you realize that with absolutely NO desire for money or power Blight was readily willing and capable of brutally murdering and torturing every living thing on Earth then casually 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!