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Nightmare Fuel: Dragon Ball
Tao Pai Pai, the most evil villain in Dragon Ball, certainly is Nightmare Fuel. Especially considering he is the most feared man on the planet Earth. In addition, he is a sadistic mercenary who is even willing to kill children.
The most chilling in Tao is the fact that he is completely human. He is not an alien (like Frieza, Vegeta or Babidi) or born for being evil (as King Piccolo, the androids or Majin Buu), he is human and evil by choice, and therefore, he is completely aware of the suffering of others.
The scene where he kills General Blue is somewhat disturbing, too, note He does it with his tongue
Then in Dragon Ball Z, Tien Shin Han has a flashback where Tao enjoyed mistreating him as a child when he was training, hinting how cruel he was, even if we talk about a flashback.
So you're watching the last few moments of Goku's fight with Piccolo, all is right, Goku gives a thumbs up to all his friends as they shout the countdown. Right when they get to nine, Piccolo gets up and fires a mouth beam straight into Goku's right shoulder. And this isn't Z where a beam to face results in some external wounds, maybe your clothes would tear or something like that. No. This is Dragon Ball and Goku now has a hole he can stick his hand into under his shoulder. After writhing in pain for a while, Piccolo gets up and begins stomping on the wound over and over as Goku grabs the ground in pain. All the while, Piccolo is grinning ear to ear, and even laughing. Tenshinhan, Yamcha and Krillin try to help but Piccolo stops them before they can enter the ring. Goku, the badass, still manages to get up and get in an attack before a swift kick from Junior reminds him he has a giant hole in his body. Junior, the prick, then elbows Goku, right in the wound, followed by more blood curdling screams. And just to add injury to injury, Piccolo breaks Goku's leg.
Speaking of Piccolo, the original Demon King manages to give a No-Holds-Barred Beatdown to Goku so badly that it stopped his heart.
Major league Adult Fear territory: Everything that happens to Gohan through the Saiyan saga - especially if you're Chi-Chi. First kidnapped by a stranger, then rescued only to be kidnapped again by someone Chi-Chi would have known for a fact was Goku's mortal enemy. He then disappears for an entire year, only to return just in time for him to be thrown into a fight with two more guys that are even worse. Chi-Chi gets mocked on occasion for her coddling ways, but after a year like that, who could blame her for being a bit rattled?
The unique attack Freeza used to kill Krillin. It involves shooting a benign (at first glance) energy ball about the size of a pinprick into an enemy's chest cavity and then expanding it whilst still inside them. Never mind the implications of your body being invaded by anything against your will, but consider that Freeza's energy was probably burning away at his insides. And you can see Krillin's chest heave unnaturally right before he explodes. At least some of Freeza's other victims got a much quicker death... some of them.
On that note, anyone who was unfortunate enough to be on a planet at the moment of its explosion.