This is not your average fairy tale
Lighthearted as the series seems, rest assured the story is chock-full of nightmares and horrors due to story being centered around a nationwide war between mass-murdering magical creatures with the theme of War Is Hell
being a prevailing premise. Not to mention the gratuitous death and violence that spares no one, not even the main characters.
From the manga:
- The state that Zaratras's corpse is found with all the weapons sticking in him and propping the corpse up. Killing him once was obviously not enough.
- When the Armored Giant is finally released from Gowther's armor, it's revealed to be a former Holy Knight whose body has been horrifically mutated. It is can be best described as a fleshy creature with four demonic heads and hands whose fingers are actually complete human arms. When the first attempt to kill this thing fails, its arms rip apart to double their number and its heads merge into a single four faced single mouthed one. Smack in the middle of this all is the original host's head which looks almost lifeless as if the whole thing surrounding him is just one big uncontrolled tumor.
- After getting hit by Gowther, Dreyfus wakes up to what appears to be a memory of him and Hendricksen when they were younger. Things however are not all what they seem. A hanged man can be prominently be seen. The town is filled with human faces situated on the walls and the ground. Characters start saying strange things such as children declaring that if there isn't gonna be a Holy War, they'd be the ones to start it. Copies of Hawk are seen littered throughout being mutilated and eaten. One of the foods Dreyfus is even served with is what looks like a human faced caterpillar. Then all of a sudden, Zaratras' corpse still in its Human Pincushion state confronts Dreyfus and begins to bleed all over Dreyfus from its eye sockets and mouth. All the while, Dreyfus repents that he didn't mean to kill his brother as he slowly transforms into a small groveling child. At the end, Gowther appears towering over the child Dreyfus and it's revealed that the Sin had basically Mind Raped the Great Holy Knight via his power over illusions.
- The way that Matrona slaughters her enemies with earth spikes is incredibly gruesome and is shown in full detail as men are skewered in droves.
- In the Side Story 2 we found out that the reason Helbram hates humans so much was that Helbram and his fairy friends met a group of humans who kidnapped them and slowly tore out their wings for money, and Helbram was forced to watch his friends die one by one. This caused Helbram to snap completely and grow a misanthropy for humans so strong he went on a killing spree against humans for 500 years straight. As a sick form of remembrance, he ends up transforming his appearance to the person who killed his fairy friends as a reminder of his hatred.
- The Armored Giant mentioned above? All of the 2nd generation Holy Knights start transforming into similar horrors in chapter 85.
- Worse is from Jericho's point of view. She joined the Knights, and became a 2nd-generation Holy Knight, to prove herself to her brother. When she transforms, she ends up nearly killing him. Jericho snaps out of her fury just in time to see her own transformed body crushing her brother to death, and there's no way for her to stop it.
- In the anime, after he has consumed the blood of a Gray Demon, Hendrickson uses an attack called Dark Snow, creating a slow-moving shower of dark spheres that instantly kills whoever it touches. When two knights remark that the attack is impressive but easy to avoid, Hendrickson tears both of their heads off in a single move while they are distracted.
- The Ten Commandments. Just one of them is more powerful than all of the Deadly Sins combined, while being severely weakened, and there is ten of them in total.
- Now add into the equation their insane regeneration, soul-sucking magic, shadowy abilities, and combine that into 10 elite demons under the will of a Demon King.
- By the way one of those demons easily killed their former leader, and managed to break Ban apart who is an immortal. Yeah scary people...
- At one point during his "fight" with the heroes, Galan impales several knights (one through the head) like they were shish-kabob.
- In Chapter 156 the hole showing the missing villagers.
- The earth crawler first seen in chapter 157. As Gilthunder describes them, and their sandcrawler offsprings, they are very territorial creatures that are known to devour anything that enters their territory, and since Droll did the "favor" of building part of his maze right on top of their territory, you can imagine how riled they must be. The earthcrawler is essentially a gigantic sand fish with a dinosaur head, whose open maw is big enough to tower over Diane. The worst part is that we see the monster devour some hapless adventurers, and the author is nice enough to let us look into its mouth and see the men's maimed bodies falling down the utter abyss that is its throat.
- Chapter 177 is this incarnate. Estarossa proceeds to impale Meliodas with one of the seven swords he summoned, while explaining to Ban that superior demons like the Ten Commandments have seven hearts, which must be destroyed before the demon himself dies. Ban tries his hardest to stop Estarossa from killing Meliodas, but all his attempts are futile and Estarossa is not even paying him attention while impaling Meliodas with the rest of the swords. Even when Ban tries to break his neck and uses Hunter's Fest to steal Estarossa's strenght when that fails, the demon continues on like nothing's happening, despite Ban getting even stronger than when he stole Galan's strength. So, when Estarossa finally impales all seven swords on Meliodas, he explodes Ban with no effort, and proceeds to say that Britannia is now theirs to take. And even when Elizabeth shows up, she can't do more than just cry and hug Meliodas, because while she has Healing Hands, she can't use it on the dead.
- Chapter 195 holds a bit of nightmare fuel when you consider that Meliodas seems like a whole different person while fighting Fraudin. The twisted smile makes him appear more like a villain than a hero. Worse still, in Chapter 196 we find out that this is a side effect of his curse: to never die but to revert to his old personality bit by bit until he ceases to be the lovable, cheerful Meliodas. The old Meliodas wanted nothing but murder and mayhem. The sheer though can send shivers down one's spine.
- Chapter 219 reveals Meliodas's brother Zeldoris has revived and unlike Meliodas he was stone-cold ruthless right from the beginning and is a strict No-Nonsense Nemesis who cuts to the chase. He immediately notices Merlin spying on him and launches an attack that hits her through her magic crystal looking glass.
- We are finally given a full image of the Demon King, and boy does his title fit his appearance. His size is more than ten times that of Meliodas, and he looks extraordinarily threatening. Meliodas even refers to him as a GOD.
- Knowing that two mortal enemy deities were actually driven to TEAM UP with each other and torment their respective children with the single most hellish curses imaginable that they had the power to inflict just because they decided to hook up. What the actual f$$k?!
- Chapter 230 gives us Ban's grisliest injuries yet... melted by poisonous digestive acid. Flesh dissolving off his bones and eyes dripping out of the sockets... Sweet dreams... NOT.
- Hey you know Estarossa? Well he isn't actually Estarossa, his actual name is Mael one of the 4 arch angels. He was once a kindhearted if ruthless warrior who got so thoroughly brainwashed into thinking he was Meliodas' brother that everyone in existence thought it as well. When he gets his memories back it isn't pretty, it actually downright horrifying since he could see himself the whole time being brainwashed and could do nothing but watch himself fall from grace and slaughter thousands of his own angel brethren and innocent humans.
- It gets even worse Mael starts trying to absorb Commandments from the other demons and literally goes through the most unsettling display of Sanity Slippage in the series with some transformations into an unstoppable babbling Humanoid Abomination to boot.
- Chandler. He's Meliodas former master who taught the guy everything he knows about combat and appears a kindly old man, but once he's pissed he turns into the nearly unstoppable bloodthirsty demon he's known to be. To put it into perspective, he came into Britannia drastically weakened and about a quarter his real strength and single-handedly decimated the Sins, manhandling Merlin and her spells with ease and easily knocking around the other Sins like flies. Even when he was distracted and had a whole blasted through his torso, it only caused him to get serious, put on his Game Face, and then go about killing Drole and Gloxinia, the respective World's Strongest Man and founders of the Giant and Fairy Clan at the time, with hardly a scratch when they got in his way. Not to mention the fact Chandler can use upgraded versions of Meliodas' own attacks and renders Escanor useless in a fight by nullifying his power with his very presence.