The very premise mentioned above is quite disturbing. The only means of combat is a maneuvering gear that has a chance of messing up as well as blades, amplifying the terror of being close to a titan.
Chapter 10: We see how Eren got out of that Titan's stomach. But first, we see Eren being eaten alive. He goes into the Titan's stomach where parts of people are floating. Someone else is even still alive and cries out for her mother. Eren still refuses to give up. Luckily, he transforms into a Titan and breaks out.
Arguably, even worse in the anime. The cavernous and extremely red insides of the Titan that ate Eren (as opposed to the black-and-white of the manga) is absolutely Hellish. It also illustrates the truly horrifying results of being eaten whole by a creature that has extremely high body temperature and doesn't actually have a working stomach: you are boiling in your comrades' blood.
The abnormal Titans are scary as hell. The Female Titan shows us exactly how easy it is for a Titan to kill a human, taking out at least a dozen in view including an elite squad with several hundred kills between them. The Colossal Titan is a 60 meter giant with no skin, with a permanent Slasher Smile. The Armored Titan is a seemingly unstoppable juggernaut with a skull-like face. And the worst part is, they're all humans too, walking among the cast unknown.
Ymir, aka the Dancing Titan, is smaller than the rest of the abnormal Titans, but looks far more feral and is much faster, like a rabid demonic animal.
The Beast Titan is unsettling. It is the only Titan to look like an animal. It is also the only Titan so far able to clearly communicate to humans. Not to mention the disturbing implication that it has the ability to turn humans into Titans.
Eren can be pretty creepy at times. He was only 9 when he killed two grown men to save Mikasa. He just cries for a while in the entrance of their hideout, pretending to be lost, and then stabs a guy right in the throat, thanking him for the lesson on how the world works. Then he just gives the creepiestglare at the other bandit, as he closes the door!
, but immediately after, Eren is shown grinning in a really maniacal way, whispering about how he'll kill all the Titans. It's downright chilling.
Let's not forget Mikasa. Anyone that so much as threatens Eren will be subjected to a Death Glare and the very real possibility of violent revenge. When Reiner moves to fight Eren, she responds with devastatingresults. And she later admits that she was holding back and hesitated. Yes, that was Mikasa when she wasn't completely committed to her actions.
In general, any time someone gets eaten by a Titan. The very idea of being Eaten Alive is already terrifying enough, and the scenes are usually extremely gory, too.
The scene where a Titan casually holds a grown woman by her leg, slowly tearing pieces of flesh from it all the while she is still alive and her fear-stricken child sits in a corner, completely frozen. The unsettling atmosphere of the scene made it far worse than most occasions where people get swallowed whole or bitten into two.
The Titans eat for pleasure, and only for pleasure. Despite how they all have mouths full of horrifying teeth, chewing is optional. It would almost look like they're playing with their food just to show off for the survivors... if their faces weren't such glazed-over masks of orgiastic bliss, completely ignoring everything but the struggling bodies going down their throats.
Titans themselves, even without the whole eating people trait, almost always have varying degrees of Body Horror. The lucky ones look a bit malnourished or have heads too big. Some others are clearly deformed beyond all reason.
The very concept of Titans, thought of from their point of view. It's one thing to look up at a Titan and think about how big it is. It's another thing entirely to think of yourself from a Titan's point of view and realize that you barely classify as an insect.
Titans, not just because the basic idea of a race of deformed-but-identifiably-human giants that eats people is terrifying, but because nothing about them makes any sense. They derive no nutrition from eating humans because their digestive systems are vestigial, but they don't eat anything else, they regenerate at an alarming rate as long as one piece of their body stays intact, no one knows how (or if) they reproduce without sex organs, they're devastatingly fast, especially for such large, ponderous creatures, and no one knows where the hell they came from.
In general, the Titans are a little bit smaller in the manga. You'd think that would be of some comfort as far as fighting them might go, but... all it really means is that most of them have to bite you in half.
The fact that Titans are primarily diurnal is also quite terrifying. Though it's a justified case, since they seem to extract most of their energy from the sun, it's still very unsettling because you can see that the world isn't some crappy and desolate place, but a beautifulone. The world itself isn't a bad one: the Titans make it bad. And the fact that a person's last sight before they get eaten are gorgeous snow capped mountains and rolling valleys is not only horrifying, but depressing.
Even during 100 years of peaceful living, the fact that Titans just hung around the Walls like a bunch of vagrants is creepy as hell.
The way the Female Titan kills some of the Survey Corps members is downright cruel. Most of them get crushed underfoot or otherwise smashed by her limbs. Some less fortunate victims die an even less dignified death, varying from having their spinal column broken against a tree, to the upper half of their body grated and smeared out across a tree's surface and being spun around by their 3D Maneuver Gear's cable, reduced to a shower of blood due to the sheer force. To call her style merciless would be an understatement.
One unlucky soldier arguably gets the worst of it. Ever twirl a yo-yo? That's what the Female Titan does to him, and the results are graphic, if brief. His spine is snapped in half, the force of him spinning is forcing all the blood out of his body, and if that's not enough to kill him, he's likely suffocating even before she decides to let go and send him rocketing off skyward.
In Episode 7, there is a rather chilling scene of a group of soldiers trapped inside a building in Trost with the Titans slowly working their way in. This alone would be cause for terror, but what truly makes unnerving is the way one of the soldiers cheerfully smiles as he loads a pistol and stuffs the barrel into his mouth.
Episode 13 manages to make Marco's death even more chilling than in the manga. Not only is the way they portray his body unusually grisly (even by the anime's standards), the monotone and wide-eyed method of addressing Jean as used by a female surveillant is almost equally disturbing.
The pile of regurgitated human remains is even creepier animated. At least poor Marco was lucky enough to be identified.
Chapter 38 can be summed up as "Somebody give Connie Springer a hug!" Just put yourself in his shoes for a second. You visit your old home village that was destroyed by Titans. Bad enough already. But, then you find that the place makes no sense. There's no blood or corpses, so everyone must have run away, but the horses are still tethered in the stables, and everything is just quiet. Too quiet. And worst of all, there's a Titan with deformed limbs on your house, one that looks strangely like your mother... and it welcomes you back.
The piles of regurgitated, undigested limbs, and the fact that it is somebody's job to clean them up. The waste of so many lives for no apparent reason is bad enough, but every soldier who has to mop up Titan vomit can't not realize that they're elbow deep in what's likely to be their own future: a pile of anonymous body parts clogging the gutters.
That... THING that Eren created to stop him, Mikasa, and Armin from being killed by cannon fire, which can only be described as a giant skeleton with skinless flesh attached to half of the bones.
That anime-eyes Titan in Episode 8.
We see it first in Episode 5.
It's actually one of the first Titans we see that doesn't have a perpetual Slasher Smile.
It may all just be an act, but the No-Holds-Barred Beatdown that Levi gives Eren at his trial can be a little hard to watch. It doesn't help that the first blow knocked a tooth out and he didn't stop there. Granted, the tooth grew back, but still.
The scene where Hannah is trying to revive Franz might have flown over some heads in the anime thanks to the view (if you don't look too carefully at the scene, it makes it look like his lower half is still there). However, in the manga, it comes as quite a shock since it's smack-dab in the middle of the page and completely unhidden.
If you do notice it, it is much more chilling in the anime by how slowly it sinks in. You see her trying to revive him, and then ask Armin for help. His refusal at first seems strange... then you see the blood... then notice Armin's expression as he refuses. It suddenly clicks, but you can't be too sure. You keep trying to see his lower half, but each camera angle shows exactly the same portion of his body as the one before. It can be a very stressful scene to watch.
The Female Titan is absolutely terrifying, both in-universe and to the audience. Not only is her appearance pure Fan Disservice and Body Horror, with exposed muscle and bone over what SHOULD be an attractive female figure... but she shows just how utterly terrifying an intelligent Titan can be. She's incredibly fast, able to easily run down the soldiers' fastest horses, and relentlessly chases down her prey while tearing through anyone in her path.
It doesn't help that she racks up a massive body count, apparently wiping out the ENTIRE right flank of the Survey Corps, considered to be Humanity's strongest military division, and by the time it finally catches up to the Levi Squad, the cream of the crop, every last one of them except Levi himself is panicking. Even Eren himself is momentarily terrified.
A flashback reveals that in the middle of having tea after failing to activate his Titan powers, they spontaneously activated and formed a fleshless arm and torso. It is part unnerving because it shows just how little Eren understands his own powers, and part because of how viciously the Levi Squad reacts to it. Then again, the mood doesn't last long as Hanji charges in, practically squeeing at the sight.
The roar the Female Titan let out was pretty damn unnerving. The massive wave of Titans that came charging in as a response to it was even worse, though.
The Colossal Titan and the Armored Titan, responsible for countless deaths, were just kids. And even up to now their motives remain unknown.
Ymir mentions being a Titan for over 60 years, and that it feels just like one big nightmare.
The reveal of the Titans in the Walls. As a piece of the Wall broke off when the Female Titan was climbing it, Mikasa sees a very large eye. It's revealed to be part of an Ancient Conspiracy that only the top of the Wall worshippers and the royals knew, to subjugate the masses.
Chapter 49 was mostly awesome, but it becomes terrifying in the end when Eren and Mikasa come face to face with the very same Titan that ate their mother five years ago, complete with that familiar grin.
Reiner chucking entire Titans at the Survey Corps in a desperate attempt to stop them is pretty unnerving, too.
Chapter 50. Nearly all of it. The Smiling Titan that ate Erenís mother returns, and the first thing it does is tear Hannes in half and eat him before going after Eren and Mikasa. The Survey Corps and the Military Police are being snapped up by the dozen, and Armin is frantically trying to protect Jean with only a single blade to raise in defense against approaching Titans.
Expanding on Hannes' death, it's rather unsettling how he opens his eyes to look at Eren when the latter starts laughing, apparently still having a bit of consciousness left even as he's torn in half.
Eren has the ability to control normal Titans and if Reiner's thoughts are anything to go by... it would be one of the worst case scenarios.
Considering the person thinking this is an enemy, this comes off as an in-universe villainous version of Nightmare Fuel. After all, the Titan Shifters can't fight an endless swarm of normal Titans. And considering how much Eren despises them...
Armin realizes Annie was the Female Titan all along. Mikasa decides to kill her. The Female Titan's reaction is to laugh and start blushing with a smile. The creepiness of this is aided and it is made even more surprising by the fact that she maintains a stoic face throughout the rest of the anime, never smiling even once (until now).
In Episode 24, Eren desperately tearing into his hand with his teeth in an attempt to transform. His hand and mouth are gushing blood, and those sounds he was making are awful to listen to.
And when he finally does turn into a Titan, it's part awesome and part this. While Eren has lost control in anger before, here he goes absolutely berserk with UnstoppableRage. His crazed, twisted facial expressions and the fact that he's covered in blood only make it worse.
Episode 25: Eren goes berserk, restating his vow to kill all the Titans, complete with a voice that sounds like he borrowed it from the Zerg Overmind and a grin that would make Alucard proud.
Everything in Episode 25 is terrifying. The fight between the Female Titan and Titan Eren leads to the destruction of the city and the deaths of many civilians who are caught in their fight, including children! Worse, it's a berserk Eren whose responsible for all this, all for his single-minded goal to kill the Female Titan, who is not only horrified on what he has done, but is also so terrified of him that she desperately tries to escape.]]
A recently released prequel comic shows a cult opening one of the outer gates seventy years before the events of the main series and a lone Titan gets in, killing a couple dozen people, maybe more. Not only is the Titan much fatter than usual, but the uniforms and equipment of two soldiers who respond to the situation heavily imply that 3D Maneuver Gear hasn't been invented yet... meaning if the Titan hadn't been lured out by people sacrificing themselves, it could have very well have been impossible to stop. And that's without getting into how he swallowed a pregnant woman whole, and how the baby survived his mother's remains being thrown back up.
Let's not forget that said chapter opens with a severed head falling from the sky, complete with a zoom in on the deadened expression and plenty of blood.
Chapter 51 has this in spades.
The Titans invading Wall Rose are confirmed to have been the missing residents of Ragako village. Connie has lost every single person he knew growing up, as they were killed by the military after being turned into Titans. His mother has been taken into custody by the Survey Corps, and may exist only as a test subject for further experimentation. The photograph of his parents also reveals the Titan that Sasha fought off in her village was Mr. Springer.
The veterans of the Survey Corps realize that because Titans are humans transformed against their will, they have been experimenting on and killing innocent victims this entire time. Of course, the killing part could be interpreted another way.
Normal Titans do not have a "pilot", as Shifters do. Instead, their bodies have been completely absorbed by the Titan form with only their spinal cord remaining. Titans can be killed through slicing out the nape of the neck because they are decapitating what remains of the human.
Anyone could become a Titan. ANYONE. It is unknown how the Beast Titan transformed the villagers, or if there are multiple methods to induce transformation. And it seems that most can never become human again, once changed.
The above is only made lighter due to the idea that Ymir was once like those Titans, but managed to return to a human form
The refugees from Wall Rose were forced into the ancient Underground cities beneath the Capital with only a week's worth of rations. They came within moments of open and violent rebellion, barely avoiding the Civil War that Pixis warned about early on in the series. Humanity is hanging on by a thread, one disaster away from slaughtering each other.
The first OVA, Ilse's Notebook, adapts the homonymous side story in the manga, but adds extra material to it. Ilse's death is spine-chilling; her head is bitten off with full of view of her body instantly going limp afterwards. The aberrant Titan then stores her headless corpse in a tree and the Survey Corps discovers it a year later. It is a rather grisly sight.
You know how the Walls are the only things keeping the Titans from eating all of mankind? There are Titans in the Goddamned Walls.
And as terrifying as that concept is, even scarier are the questions it raises. Who put them in there? How did they get them in there? And most terrifying of all: Why? This is a revelation that turned the entire series on its head, due to all the disturbing implications and questions it raises for the future.
There's some pretty scary implication behind whoever or whatever the boss of Reiner, Annie and Bertolt is. Imagine the mindset of people sending three kids to exterminate humanity. Made worse when Bertolt confesses while crying that he genuinely cared for his comrades, but he's too scared not to follow orders, and Annie would rather seal herself away in crystal (possibly forever), than give up information.
Chapter 53 has quite a bit of horror in the beginning.
Eren's Titan form in the first pages is malformed. In which the Titan doesn't have flesh from half of his spine down. You can see Eren's human body attached to the Titan's neck. Hange notes Eren is permanently fusing with his Titan form before they get him out. When pulling Eren out of the body, it's possible they ripped off his face when trying to free him, as we see his skull.
There's also Armin being molested while posing as Historia's Body Double. Armin's terrified expression and Jean having to look away plays up the horror of the moment, in what might be played as comedy in a different series.
Chapter 57: The antipersonnel 3D maneuvering gear. We're seeing the weapon that's supposed to be used for killing Titans, now modified and used for the sake of killing humans. It's quite chilling to see it in action as the gear was used in killing off Hanji's squad with startling speed and accuracy before they even knew what hit them. Even Levi was struck speechless for a few seconds in seeing the damage.
Also, there's that matter of Rod Reiss' plan to EAT Eren and gain his coordinate and his Titan shifting powers, as Ymir did to Berrik. One can only imagine how he plans to do so...
Well, either that or feed him to one of the Titans within the Walls. Still horrifying.
The possibility that Titans don't want to eat humans, but that they are driven to in order to possibly eat a Titan Shifter and escape the mental hell of being a Titan. Whoever or whatever designed the Titans to mass murder humans did it in the cruelest way possible: by insuring that everyone's a victim.
Eren and Historia being confined and transported around in coffins would look pretty unnerving to anyone with the slightest case of claustrophobia.
The Military Police, due to its irredeemably corrupt nature and its constant oppression and lying to the people cowering within the wall and endless tormenting of the Survey Corps who are fighting so hard to save the humanity from the Titans without being given even a word of gratitude from the people they are trying to save. It is completely impossible to argue whether the MP or the Titans are worse.
In Chapter 58 it's quite possible that Jean was shot in the head in front of all of his friends by a anti-human personal gunman that he was trying to not kill in the first place.
Chapter 59 reveals that Armin shot the woman, sparing Jean's life. However this good news comes with a clear shot of her bloody and clearly shattered skull from the impact of his bullet. Worse still, Jean explains that the reason for his hesitation to kill her was that she was hesitating to kill him. It may have been possible to talk her down, but of course no one could risk it. For readers upset with Jean for not just shooting her, it is an unsettling revelation to say the least.
Towards the end of the mostly awesome chapter 61, we get Rod Reiss's creepy grin. It's especially jarring, since the only expressions we've seen him wearing so far are either grim seriousness or apologetic worry. Worse, this face is shown as he and Historia stand before a chained and bound Eren. Eren's position and the shot of Kenny sharpening a knife imply they are moments from doing something horrible—and the Survey Corps aren't even close to finding him, having only vague clues to go on.
So it turns out that Eren's father didn't make him a Titan shifter, the injection initally made Eren a regular mindless Titan, and, hard as it is to believe, mindless!Titan Eren looks EVEN CREEPIER THAN THE SMILING TITAN.
What happened after that is even worse. Eren's father was a Titan Shifter, and when he turned Eren into a Titan, he was promptly attacked and bitten in half, granting Eren his shifter powers. Eren immediately turns back to normal, and the first thing he sees is his father's mangled corpse.