Death Note/Nightmare Fuel

• The deleted scene where Light stays in the graveyard after L's funeral, starts laughing madly, proceeds to crawl on top of L's grave and shouts "What do you think of that, L? This is my perfect victory! That's right--I win!" with a crazed look on his face... and looks suspiciously like he's humping L's grave. See for yourself.
• Basically every time Light goes into crazy Kira mode, with the maniacal laugh and the red eyes.
• L's death. The poor guy suddenly collapses from a heart attack and the last thing he sees before he dies is Light staring at him while giving him a huge Slasher Smile.
• Light's death. In the manga, it's even worse. you see his dead face on a two-page panel, with a horrifying expression, which is worst up-side-down.

Light: NO! STOP IT! I'M GOING TO DIE! I DON'T WANT TO DIE! I DON'T WANT TO DIE! I DON'T WANT TO GO...

• Light and Misa in and of themselves... they are adorable... (in some of the early manga artwork Light kind of looks like a Disney character) and well, you know...
• Mikami demonstrates the anime rule of "the human body contains 67 gallons of blood and more under high pressure."
• Mikami was just as scary in this scene, where he goes into a delete-spree.
• Light and Mikami's facial expressions in the last two episodes. They look absolutely demented.
• The moment Light moves from saying "I'm going to create a perfect world" to "I've gotten rid of everything in MY way." This shows he has Jumped Off the Slippery Slope into true evil and has absolutely no chance of redemption from here on out.
• Takada's death, where Light callously lets her burn alive after having served her purpose.
• Naomi's death, particularly if you hold to the theory that people under the control of the Death Note are still capable of thinking independently, effectively rendering them prisoner to their own bodies.
• Some scenes from the second Rewrite (L's succesors):
  • There's an altered version of the scene where several SPK members get killed, except this time by Mikami instead of Mello. Here they put some really Uncanny-Valley-ish expressions. And one attempts to kill Near with his gun. The final shot shows Near staring silently to the screen, covered by the blood of the corpse that's right next to him.
• The scene where Mikami is introduced. He intervenes in a debate on TV about Kira by slaughtering the audience in various gruesome ways (watch from 1:09). One of them gets his neck twisted in 180° degrees. By the way, their faces get even more Uncanny-Valley-ish.
• Light Yagami. Or, rather more accurately, what Light Yagami becomes. He starts out as an innocent idealist (as seen in the Yotsuba Arc) and slowly becomes more and more ruthless, horrifying and generally evil, until he's reached the point where he's only barely recognisable as the same person he was.
• Teru Mikami—or, how an otherwise decent person can go completely off the deep end. The part where he spins around in his chair and laughs is the worst. Good choice, Light.
• Misa Amane and Kiyomi Takada—women completely devoted to a manipulative sociopath and willing to kill for him. All the scarier because it can, and does, happen in Real Life.
• Light's voice whenever he details one of his plans. Also, the way he looks down at the dying Raye and says "Goodbye, Raye Penbar" in that mocking tone. (Ditto for when he says basically the same thing for Naomi Misora.)
• The Yagami family. Sayu has a brother who is perfectly willing to kill her if he thinks she's getting too close to uncovering the truth. Light even suggests, early in the series, that he is willing to write down his junior-high-school aged sister's name in the notebook if she so much as sees it.
• Misa Amane wears an Elegant Gothic Lolita style dress in one instance, for when she was committing suicide after she thought Light died. Overall, the dress combined with her depressed facial expression and her dark make-up makes her look like a life-sized porcelain doll, to an eerie effect. That scene mirrors an earlier scene where Misa, dressed the same and traveling through the same setting, sings a capella about her devotion and trust to Kira. Though the lyrics are optimistic, her tone, and doll-like dress, makes the scene still quite creepy.
• In the very first episode, Light locks himself into his darkened bedroom, pulls out his Death Note, opens it up to reveal all the pages and pages of names of the people he's killed, smiles down at it, and then breaks out into creepy, creepy laughter.
• Misa in custody. Imagine the mental state of the poor girl. She's taken off the street, continuously blindfolded and restrained for two months, under constant surveillance, and she has no idea why she is there or if she will ever be untied.