Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Future Diary
If you don't find Yuno to be very endearing and Moe, you'll most likely find her to be one of the scariest, craziest, most deranged girls in anime. Here are some examples:
Her initial introduction. It's made very apparent that she stalks Yukiteru. The one part that stands out the most is when she's looking through the mail slip in the door.
Her digging a massive hole in her house while singing about Yukiteru... then forgetting why she was digging the hole at all in the first place.
Her expression when Yukiteru finds the corpses in her house.
What about her infamous face at the end of episode 1 of the anime adaptation?
Anytime she has an ax or sharp object on hand.
The crowner for this is when she first comes to the realization that everything will be okay if Tsubaki just disappeared. The pleased-as-punch look on her face as her alignment shifts permanently to Chainsaw Good is downright unsettling.
When she drugs and locks up Yuki with the intention of keeping him safe and together with her. Forever.Shudder. Of course, this is after we learn that she killed her parents.
When you find out that this Yuno is from an earlier timeline. She's essentially gone back in time and killed herself so she could be with Yuki. There is nothing this girl won't do.
Then there's the implication that Yuno was going to torture Yukiteru's mother with her "tools" (weapons) unless she approved of Yuno as a potential bride for her son. She says to herself at night while giving a creepy unblinking stare in bed in the anime that it's a good thing that Yuki's mother was so nice because that way Yuno didn't have to use her "tools" (weapons) to persuade her. Also there was her suddenly opening her eyes in the anime after taking a shower and going into a Troubled Fetal Position. The music made it even creepier.
The idea that Yuno was invited into the game of death together with the one she loved, killed him and everyone else and became god, only to finally reverse time to before her becoming god for the sole reason of a few hours alone with Yukiteru, only to later kill him and do it again is more scarier than anything else she had ever done in this series. Capable of making the rest of eternity be about a time loop where she kills everyone for the sake of her loved one over and over and over?
Yuno allowed Yuki to kill himself so she could become God and bring him back to life. When this failed she jumped back so she could be with him again for two-three months, killing all the while. When Yuki once again refuses to kill her, she intends to kill him and go back to do it all over again. Presumably indefinitely. Luckily this is only the 2nd-world.
Deus is kind of scary, but... you know a girl is scary when she's the only entry in a series which features an Eldritch Abomination.
To put this in context count the number of articles about Yuno on this page. Now count how many AREN'T.
A particularly disturbing mention must be given to Yuno in Chapter 49/Episode 22: Akise has stabbed Yuno and left her for dead, and is trying to reach Yukiteru before he kills the Eighth and Hinata/Mao/Kousaka. Yuno, bleeding to death on the ground, calls Yuki and tells him that her diary told her that they were going to betray and kill him. Yuki, in the middle of a mental breakdown at the time, believes her and shoots (and kills) Hinata. Yuno, on the other line, starts laughing her butt off, while a shot of her diary reveals that, no, Yuki's friends weren't lying. She just didn't want them to have him.Good Lord.
Yuno's cheerful smile in Chapter 50/Episode 22 after stabbing Akise. There's also her furious expression in the same chapter/episode as she's watching Akise kiss Yuki and how she tells Akise completely seriously "You die here and now!"
In the following chapter there's also her slicing Akise across the throat and later decapitating him. Also counts as a Tearjerker.
The part where it's revealed that Yuno's control-freak parents, who measured everything she did from how many hours she got to sleep to how many calories she had a day, used to keep her in a cage and starve her in an effort to raise her to be a model person. To top it off, she eventually kept them in the cage when she had enough of their treatment of her and starved them to the point where they died.
What's worse is that it started out as a lesson. "This is what you were doing to me. If you promise not to do it, I'll let you go and we can be a happy family again." When they spurned her, she just left them there.
Yuno now keeps her parent's corpses in a cage and talks with them. Later, she chops off their heads and carries them around with her.
When Yuno deliberately stabs herself in the torso.
Yuno has a unnerving habit of staring creepily at people. One notable scene◊ is when she finds out Kurou Amano is trying to find and destroy Yuki's phone which will kill Yuki in the process.
When Yuno first meets Yuki she surprises him with a kiss. It's creepy rather than romantic.
Chapter 53/Episode 23: Yuno suddenly sitting up and spitting out the pills after she had supposedly committed a 'lovers suicide' with Yukiteru.
First-world Yuno killing 2nd-world Yuno and saying "I don't need two of me."
2nd-world Yuno writing "Help me" on the wall with her own blood upon hearing Yukiteru's voice.
In the follow-up manga titled Redial... Yuno Gasai of the 3rd-world is shown to be a normal school girl. With friends and family. Just one problem with that. She is haunted by the dreams that contain details of this story, the first Yuno's story. It make her feel like something, someone is missing... and the FIRST thing that she goes and does is begin stalking her world's Yuki
Other Diary Users
The Third Diary Owner was Takao Hiyama, homeroom teacher of Yuna and Yukiteru. He's different from the other diary owners though. While the others were mayors, terrorists, cops, or even school children, this guy was a SERIAL KILLER. Oh, and boy was he paranoid about being caught. As the serial killer he is, Hiyama wears a hat/wig thingy to hide his identity, a gas mask with googles and a bulletproof and apparently EXPLOSIVE-proof jacket. This makes him impervious to almost ANYTHING. His trademark weapon was a MACHETE. Oh, did I mention that he kept a diary that recorded his victims and decides how to best corner and inevitably kill them? He didn't get much screen time in the end, but if it wasn't for him, Yukiteru's identity as First most likely would have stayed secret for longer as he was the first one to discover him (with possibly the exception of Eleventh, but he had hax). This guy is THE killer.
Reisuke (The Fifth) is a prime example. A 4-year-old who floods the entire Yuki household with poisonous gas in order to kill Yuki and Yuno. The innocent expression he keeps through out just makes it creepier.
John aka the Eleventh's ideals: He's basically a Hitler who can see the future.
Hirasaka aka the Twelfth. OH GOD, HIRASAKA.
Kurou Amano, Yuki's dad, appears in Chapter 30 claiming to just want to check on his son, but it turns out that he has only returned to destroy Yukiteru's diary in order to fulfill a deal with Seventh. Despite his only clear desire being to free himself from his debts, he does try to save Yuki when the latter almost falls to his death, but takes the parachute away from Yuki before the tower crumbles soon after. He then stabs his wife when she tries to take him to the police and escapes, though is stabbed to death himself two chapters later by the Eleventh's men.
Yukiteru attempts to destroy Minene's cell phone by throwing a dart, but she moves the phone away from her face, resulting in the dart meeting her eye.
She gets a double whammy of Eye Scream, when she is drugged by Twelfth, and unable to fight back, has her bad eye plucked out of her head. By hand.
The third opening of the anime is downright gruesome - a naked Yuno is seen standing in an iron maiden (a torture device) just as the spiked door closes, all the diary owners are seen dying or dead, mostly Impaled with Extreme Prejudice. Twelfth, Fifth and Third are all bleeding from their faces or drowning in their own blood, Eighth explodes into a pile of blood, Eleventh dies with a goblet of his own blood, Seventh die in each other's arms whilst bleeding and stabbed, Ninth and Fourth shoot each other, and Sixth crying tears of blood in a room filled with dead naked men. And then Yuno steps towards a guillotine as the blade comes down. Tenth's bloody fate is not shown but maybe that's for the best.
A few of the OST tracks count. "Cries from Avici" is a very good example, and it's even worse when you translate the lyrics, which are all about her not fearing Hell because it's what she lives every single day. Brrr...