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 moreUncanny-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.
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's 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's 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's there or if she'll ever be untied.
When Beyond Birthday committed his third murder, he attempted an experiment. Namely, to see if it were possible for a human being to die of internal hemorrhaging without rupturing any organs. Specifically, he drugged his victim so they fell unconscious, tied them up, and proceeded to beat their left arm thoroughly, being careful not to break the skin. He was hoping to bring about enough hemorrhaging to cause death from loss of blood, but this attempt ended, sadly, in failure. Blood congested in the arm and it turned purplish red beneath the skin, but the victim did not die. He simply shook, convulsed, and remained alive. He had been convinced the blood loss incurred was enough to kill someone, but apparently he had underestimated the matter. As far as Beyond Birthday was concerned, the actual method of murder rated fairly low on the amusement scale, and it was never more than an interesting experiment. It did not particularly matter to him whether it succeeded or not. Beyond Birthday simply shrugged, and took out a knife...
From the Pilot chapter, Taro's nightmare. The people he accidentally killed return as reanimated corpses to accuse him and grab at him with their skeletal hands.
Ryuk can be this. Especially during the earlier chapters, when he's there to provide Exposition on the nature of Death Notes and Shinigami. Those eyes.....
The revelation of the Death Note's last rule: "All humans, without exception, will eventually die," and, "When they die, the place they go is Mu (Nothingness)." Especially in the anime, where Light is moments away from his horrific demise and the scene simply cuts to the rules, with no music but one rather ominous church bell funeral toll. The overall effect is damn chilling.