Superman: The Animated Series/Nightmare Fuel

Despite The DCU is known for its contrast between light and darkness, it doesn't mean that Superman: The Animated Series is Lighter and Softer than Batman: The Animated Series in any way, as the following examples of Nightmare Fuel will quickly tell you.

Darkseid, the Big Bad, is a stone-skinned Evil Overlord from a hellish Crapsack World who isn't amusing or likable or suave like Lex Luthor: he's just cold, calculating, and plain evil. While the show weakened Supes so the villain of the week stood a chance, Darkseid was one of the few to really crack Supes and inflict Clothing Damage. Darkseid after he is all beat up himself isn't a pleasant sight, either.
- Granny Goodness, a sadistic hag who raises orphans to be superpowered killers loyal to Darkseid through torture and brainwashing. And she's voiced by Edward Asner Ed Asner, of all people.
The Toyman is portrayed in the episode as completely disturbed, wearing a porcelain mask, sporting a creepy child-speak, and wielding genuinely dangerous weapons. Said weapons include a bouncy ball that gains in speed until it ricochets so fast it can crush steel, toy soldiers that fire real bullets, and play-doh that suffocates. And he always seems to come out of the shadows. His duck-like weapon is creepy as well.

Duck: (in an almost human voice) "Ruuuhwaah..."

- Fridge Horror ensues when the Toyman essentially dresses up Lois Lane as a doll.
- In the follow-up episode, he is now a Stalker with a Crush, is actually made worse in that regard, as the character named Darcy, the model, is later revealed to be a human-like robot he created as a "playmate". Lana catches Darcy removing her face and spot welding her facial circuitry to repair damage from the Toyman's latest kidnapping attempt.
The origin of Parasite: he's in the back of a truck and chemicals spill all over him, horrendously transforming him into a monster than can drain abilities and memories from other people, even Superman. At one point, he switches to Clark's voice and makes a "I'm calling sick today" call. On a related note, Parasite's screams in "Two's a Crowd" are some of the mos unnervingly realistic ones placed into a kid's television show.
Unity, the local Eldritch Abomination. Especially during one scene where the Mind Rape portrayed is way too reminiscent of actual rape.

"Give in, Kara. Don't resist. Join us. Accept us. Accept Unity."

Karkull. In the comics, he's a guy who can turn into shadow; in the series, he's basically a cross between Satan and Cthulhu. He turns the Daily Planet into a portal to hell, and at the end, as Superman goes to the bottom to catch the tablet that can re-seal him, an unseen Eldritch Abomination starts rising up. Also, when Karkull is chanting out a spell, the words he uses are real, as a literal Ominous Latin Chanting.
Metallo, a criminal who, after being jailed by Superman, agrees to be turned into a Cyborg by Lex Luthor; then, after the transformation is complete, he realizes that he doesn't have any sense of touch, taste, or smell, and it drives him mad, as he's pretty much deprived of your senses and stuck in an unfeeling metal body for the rest of his life. The part when he rips off half of his fake skin is the exact moment he realizes exactly what he has become.
The Joker's laugh in the scene for "The World's Finest"; when he's destroying Metropolis, he has a deranged look on his face.
The Stinger at the end of the three-part pilot The Last Son of Krypton. Hapless alien explorers discover Brainiac's satellite pod after he ditched Krypton like a rat escaping a sinking ship. Brainiac then bursts out of the pod, and things get ugly very fast. He murders the aliens so brutally that we actually see their blood splatter on the walls. He is cast in shadow the entire time, which just makes it worse.
The series generally stays away from any real violence, except in one memorable incident in part two of "The Main Man", where Lobo grabs a Sandworm by the tail and rips off its skin.
The ending of "The Late Mr. Kent". Detective Bowman is desperately trying to figure out why Clark Kent wasn't killed by his car bomb. In the very last scene, he finally realizes he's Superman. And then the lever to the gas chamber he is being executed in is pulled.
The brief moment in Bizarro's debut episode when he breaks open one of Lex's Superman cloning jars onto the floor and the yellow inborn Superman body slides towards Lois' feet.
The climax of "My Girl", where Superman saves Lana from being covered in molten lead in barely the nick of time, then she has to desperately get to higher ground as the factory is flooded with it.