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.
In the episode where Blight, Greedly, Nukem and Scumm took over a TV station in order to broadcast eco-unfriendly programming, one of their children's cartoons had a dog's skull being opened up, and having radiator fluid being poured into it!
One episode 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 Scumm, 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.
When Wheeler returns home in New York...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.
In one episode, 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.
Actually 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.
In "Deadly Ransom," Captain Planet is captured by Blight and Nukem. The two of them shoot down the Geocruiser 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 (3 of whom are orphans and let's not forget Wheeler's S.O.B. stepfather) 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.
In "Summit To Save Earth Part 2", Zarm shows the Planeteers a smog covered Paris and some birds trying to escape. One unfortunate bird instantly dissolve into bones.