Samurai Jack/Nightmare Fuel

• If you watch the whole series without at least one squeak of terror, then you have more gumption than Jack himself. Some episodes were entirely built around Nightmare Fuel, like the episodes featuring the trip through a zombie graveyard, the Minions of Set from Jack In Egypt, and the haunted house. Even the episodes not revolving solely around Nightmare Fuel have scenes that are simply traumatic, the frequent robot carnage that the censors let pass in every episode occasionally goes way beyond the borders of "not too bad;" once you reach dismembered robots impaled on spikes, things have gotten bad.
• Episode 15, "Jack's Tales", has Jack encounter a family of metal-eating robots. The robots, in a fit of hunger, rip each other to shreds, and the insides of the robots look just graphically enough like human flesh to creep you out.
• Aku himself suits this. Especially in his first appearance in the pilot.
  • The two-parter explaining his origin adds to his terror. Apparently, he started out as this freaky monster inside the earth that would send large, deep cracks through the ground. The cracks were filled with him as he took the form of a tar-like substance, and he would then absorb living creatures to get stronger. At one point, a man calls out for his dog, goes to the back of his house, and sees one of the cracks filled with Aku-goop, and half the dog's leash sticking out. Doubles as a Tear Jerker when we hear the man's anguished and horrified screams.
  • When Jack's father goes to stop Aku, charging into the forest of black, spiky trees with an army of samurai in tow. As they get closer and closer more trees burst up from the ground, and we hear samurai start screaming as their horses (now riderless) run by Jack's father. And then, after Jack's dad wakes Aku, Aku ties him up, slips into the ground, then pops back up by his kingdom and attacks it. Coupled with the guy's horrified face and the Big No that follows, imagine how the people in the city felt.
  • Even one of the most hilarious episodes where he's involved merely paints him horrific in a different light. He rounds up kids from within a capital city, with no reason given to them or their parents. The room they're led to is large, red and spiky as always, but has monitors of adorable, yet...off animals, which eventually switch off. The camera focuses on a large red throne in the room. And Aku just appears. No fanfare, no flashy effects, no booming monologue. He's just there. This is how he reads propoganda stories to children.
• Demongo. The guy who stole essence.
• The episode "Jack and the Ultra-robots" is worth mentioning, especially the second city. Mutilated robots with horrific faces strewn about, some of them on pikes, and oil EVERYWHERE.
• "The Aku Infection". Aku spreads some evil that infects Jack and soon starts to look scarier and scarier as it goes on.

Jack: I'm getting...
(lifts up cloak to reveal Jack is looking and sounding more like Aku)
Jack: ...STRONGER.

• • Right before that, possessed-Jack carves a robot bystander's face open for bumping into him on the street. We get a very close, slow shot of this happening; never before has the Symbolic Blood looked so terrifying.
• In the second episode there are silhouettes of crucified, intellectual dogs, and it's implied that they are dead; Jack's reaction upon seeing it is entirely justified. Aku has been doing this since Jack's original time period to multiple races throughout multiple galaxies. For thousands of years. That's horrifying. It makes you wonder if the universe was better off with the original abomination...
• "Jack And The Zombies". Some of those zombies were just plain freaky, like the long-armed zombie and the one that possesses Jack's sword. And when he falls into the crypt... that wail.