This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Detective Conan
The eyes of a sadistic murderer.
This woman's face. Most people have seen the numerous dead bodies in this series without flinching, but this picture of a woman BEFORE death ...Just look, even though she's a bit demented, she KNOWS she's going to die. That expression on her face is nightmare fuel enough.
Not for everyone. Mrs. Ayane's expression is so exaggerated that, for some, it actually plows straight into Narm.
Just... how the events began, from Ran's prespective. She has a fun little day with Shinichi at an amusement park, but things get worse when a murder happens. It's thankfully quickly solved and the day could be somewhat saved, when Shinichi decides to run after two suspicious guys, non-chalantly saying that he'll be right back... and then he doesn't come back. He's just... gone! He's not at home, he's not dropping by your place or any other place he's known to be at... she can't find him. Just imagine if one of your friends suddenly disappeared like that. While Shinichi eventually "returns" and tells her he's just off solving cases somewhere, think of the anxiety she must have been going through until she finally got a proper "reason" for his disappearance.
Sunset Mansion. Ever wondered what it'd be like to see Lawful Good characters murder one another? They all live and are unharmed (save for the Hercule Poirot knockoff who was actually one of the two villains and was killed by the other one), but Hakuba and Kogoro with guns is something one doesn't want to think about. One has to note, though, if we believe in the Non-Serial MovieThe Fourteenth Target, Kogoro is actually a top-notch pistol marksman.
The 'Mountain Villa Murder' case, anyone? As in the one where the killer chops the victim to little bits and smuggles them into the forest one-by-one under his shirt, and attacks Ran/Conan with an axe in the middle of the night?
The Killer's 'Madman' persona is pretty scary on its own. A figure in dark rain gear, his face and hands covered in old bandages, with a crazed look in his eyes and maniacal grin? Brrr.....
How the victim was found wasn't much better. The victim's friends find her apparently lying on the ground. When one of the dudes tries to lift her up, her head falls off. AAAAAHHHHH!!!
The murder from the very first episode featuring decapitation and High-Pressure Blood. Yes, this episode is the only one where the Japanese censored the blood.
Blood in the anime has always been censored in Japan. This episode is noteworthy for the blood being white, but even in the newest episodes blood is never shown red, it is censored and shown as a black color.
To celebrate the series' twentieth anniversary, there was a one-hour anime special re-telling and expanding on the first episode itself. While the blood was darker, like explained above, in exchange there was more blood and it flowed from earlier - with Ran getting several blood stains on the left side of her face, in example. YEOWCH.
Also, the case that takes place immediately after Heiji Hattori's introduction case — the one where Conan and his three elementary school-age friends are stalked around the library by a psychopathic, drug-smuggling librarian who has no problem whatsoever with strangling them because they know too much? Not to mention his face during the whole time he was stalking them... in the dark.
The Rich Bitch Reika Yotsui's death in Billionaire Birthday Blues is simply made of this. She was actually bound and taped up inside a bathtub, with the cover placed over her....while it gradually filled up with water... and drowned her. And it happened in the span of hours, specifically to make Reika experience a slow and terrifying death, and it shows when her corpse is found with bulging eyes and an open mouth... meaning she was trying to scream for help for a long time, yet nobody could hear her. *shiver* She is a massive Asshole Victim, but still.
In the manga, Shinichi's mental description of what's happening to his body while he shrinks sounds rather unpleasant.
Not that it's much better in the anime either. His train of thoughts as he shrinks for the first time is by the lines of "I feel like my bones... are melting away! MY BONES ARE MELTING AWAY! MELTING AWAY!!". Eeeeeew.
It really doesn't help how desperate and terrified he sounds. In the dub, he sounds like he's about two seconds away from having a mental breakdown over his impending death.
It's sort of worse when you think about the mechanics of the process, knowing the poison is named for apoptosis. Apoptosis is a mechanism built into the DNA and is essentially cellular self-destruct, a way to combat potentially mutated cells. When you think about it, this poison is forcing every single cell in his body to kill itself all at once, quickly spreading through the entire body. Everything is dying and the heat is causing the chemicals to evaporate and the body to erode all by itself, all while the heart is beating out of control trying to keep the body alive. The whole body broken down piece by piece, cell by cell. One has to wonder if, for those that the stuff would have killed, if the poison would have left ANY cells intact, or whether it would have kept going until only a tiny shred of the body remained, along with already-dead cells like hair and nails. They said it was untraceable in an autopsy, but if the toxin doesn't level off on its own, it might not leave something to do an autopsy on. Not to mention that the gas coming off the body might be rather noxious if it contained more than water. Shudder.
Massacre Night, if you do not know how the villain went about what they did, then you might be a little jumpy, because that can be done in real life too. The spelling out of Massacre Night and everything made the killer sound like a demented serial killer. And the characters are all stranded up in a blizzard.
The Mermaid Island case seriously freaks at least some troper out. Heiji and Kazuha, dangling off a cliff, Heiji only hanging on with one hand... and she stabs his other hand with an arrow to make him drop her so he can still save himself. It goes into massive awesome when not only he refuses to drop her, but manages to hold on until they're saved.
A Cursed Mask Coldly Laughs. There's just something deeply unsettling about getting killed with your throat slit by a knife, but it gets even worse once you consider the knife stabbed the throat. As in, it was pushed forward.
In the movie Phantom of Baker Street, there's Jack the freaking Ripper. He's introduced as a shadowy figure who runs around, brutally murdering young women and who comes out of nowhere, terrorizing everyone. Then, it's revealed that he's this gangly man with horrifying claw-like hands and the most insane laugh imaginable, which he lets rip while shouting how his bloodline will survive on Noah's Ark. Oh, and he murdered his mother and kept murdering others because he simply likes it.
Imagine getting ready to visit a very dear friend (or better said, his grave). Then, you get knocked out in your way there. And when you wake up, you're caught in a Death Trap where you're Bound and Gagged on a plank placed in the middle of a construction site, with a noose around your neck so you'll get yourself hanged if you roll over... and if you don't, you'll either dehydrate/starve to death or freeze to your doom... and unbeknownst to you until later, there's a time bomb under the plank itself. And for worse? A camera has been installed over the trap itself, sending your completely helpless image to your friends and your girlfriend who are at the other side of the country, desperately trying to find and save you.And you don't even know what have you done to deserve this. Poor, POOR Takagi. (Who, for worse, was caught up by mistake!) Thank God he's saved in the nick of time.
If anything, Aoyama has topped himself in chapter 915. Remember the aforementioned first murder through decapitation? Well, 915 finishes with some poor Red Shirt from Nagano Police being also decapitated on-screen, thanks to a noose tied up to his neck and with its other end tied to a vehicle that then goes off a cliff, taking the poor guy with it and cutting off his head in its way down. It even crosses into Tear Jerker because we see the poor sap, who had done nothing utterly terrible or wrong until then, crying Tears of Fear as his death nears... (That one gets subverted in 917, but we didn't know that in these moments.) Aoyama, what the HELL.
Episode 91 (94 in the dub) "The Bank Robber Hospitalization Case" - a man is being forced by a gang of bank robbers to murder one of their ranks who has been captured by the police after a car crash. To make sure he complies, they threaten his young daughter. When he tries to talk to the police hanging out at the hospital, he's approached by a security guard, who turns out to be a member of the gang sent there to keep him in line. The sheer paranoia and hopelessness of the scene in question, especially when the poor man slides down the wall, his legs having given out from terror, is bone-chilling.
Partly Fridge Horror in the episode 829: A man who have just committed a murder is visited by a boy in the middle of the night who acts like he can read the mind of the man. Upon being exposed by the boy, he realized that he have met the child before... and that the child looks unaged from when they met 10 years ago. While we as viewers know about Conan and the context of the story, the store owner have no idea. To him, the child is like a ghost taken from a horror movie.
The two-part flashback story (921), telling how Ran and Shinichi met in preschool, is absolutely terrifying when it's revealed that their teacher was trying to groom Ran to trust him exclusively so he could kidnap her, because his wife wanted to raise her as a replacement daughter. This included slipping her sleeping pills at nap time and planning for his brother-in-law to sneak in through the bathroom window and take her, while the other children slept. Thankfully, Shinichi and his dad solved the case in time!