There is a fair amount of quite grisly deaths and dismemberment in the series that the anime adaptation kept largely uncensored. Special mention must go to graphic scenes of characters getting devoured by orcs.
The anime opens with Tokyo being assaulted with a rain of fire by B-29 bombers in 1945. A woman and her child attempt to run to safety, but in a flash of flames the woman is gone. The child is left crying out in terror for her mother... before a hand reaches out to her from a distance. The fire grows intense and encircles the child, lifting her off the ground and incinerating her.
Satoru's final moments as a human, while undercut with moments of levity, is played deadly straight at a glance. A well-meaning man takes a fatal blow for his friend on the street due to some unknown thug rushing down the sidewalk with a knife, and dies on the ground before anyone can even call an ambulance as he thinks to himself about all the things he never got to do and how much pain he's in. Imagine being someone to witness that out of nowhere, let alone experience it firsthand like our hero does. The anime adaptation shows him lying down in his own pool of fresh blood.
The actions of Rimuru and his kingdom both before and after he becomes a True Demon Lord in his "war" against Farmus are absolutely chilling. While the majority were Asshole Victims, the fact they all slaughtered at least 20,000 people combined, including people that were only fighting because their country said to, was both awe-inspiring and terrifying, especially the sheer variety with which all of them were killed. The majority got relatively merciful deaths through Rimuru frying their brains with concentrated sunlight or having their souls forcibly and suddenly ripped out, but the rest weren't so lucky, being brutally mutilated while still alive before death finally comes for them.
Halfway through the massacre, Rimuru gets the skill [Merciless], which more than lives up to it's name. What it does is it allows him to kill anyone who begs him for mercy. Given that the entire remaining army is terrified of him now, Rimuru uses the skill to quickly dispose of the rest of the army and everyone in the area (save for the king and bishop, who were spared because they're potentially more useful to him alive) just instantly drops dead.
The manga and anime took visual liberties with the whole affair to turn that concentrated sunlight into something more like a laser beam remotely controlled. The result is soldiers being killed so fast, so quickly, that holes are left in their vital organs and across their bodies before they even have time to react- or for the blood to flow. Everyone that suffered a painless death didn't even have the opportunity to see it coming unless they happened to catch sight of the water droplets. Really highlights the sheer gap in power, and that these were the lucky bastards compared to the rest of the carnage.
Diablo serves as this for Razen, and it is not hard to see why. A seemingly invincible, ancient Arch-demon who was once the rival for the World's Strongest Man, and he toys with Razen like a cat would a mouse. It may also be awesome, but you cannot help but feel sorry for Razen, who was unfortunate enough to have the undivided attention of such a powerful, nigh-unstoppable entity. It doesn't stop with Razen either, anytime he chooses to have his fun, people are gonna be terrified. Orthos's last moments were in panicked agony, the Phantom was torn to piecesbefore his throat was crushed and the three members of The Seven Luminaries that managed to earn his ire were erased completelywith their last moments in complete helplessness. During every one of these engagements so far in the anime, the ominous chanting and One-Woman Wail of "Saiseiki" plays during these fights, to underscore just how hopeless their situation is. Even when not fighting, he boasts an air of superiority and mere mention of his talents make even the like of Elalude and Gazel sweat.
While she's always been a total sweetheart that is absolutely terrifying when angry or holding a grudge, Shion becomes an absolute nightmare following her resurrection and acquiring of the "Chef" skill. Despite the name, the skill allows her to manipulate the physical structure and state of existence of anything weaker than her to follow her desires. One notable application of this is inducing Body Horror on people that have done wrong by Rimuru, with the targets forced through this being left in an And I Must Scream state, since any changes she makes with the skill will be considered "normal" by the body and healing magic, meaning they are forced to continue living as misshapen masses of flesh.
The final battle with Clayman gets particularly chilling once Rimuru stops holding back. After No Selling Clayman's best attacks, Rimuru starts punching the crap out of Clayman, but also imparts [Thought Acceleration] on Clayman so that his perception of the beatdown was increased by 1,000,000 times normal (for perspective, one second would be equal to approximately 277 hours or 11.5 days). In other words, the several seconds Rimuru spent smashing in Clayman's face was to the latter's perspective weeks to months of continuous pain, to the point that by the time it was over Clayman is reduced to a groaning shivering shell that in the light novel his hair is starting to fall out from the stress. Then there's Rimuru actually absorbing Clayman via [Beelzebub], where he promises Clayman that the instant it takes in real-time will feel like an eternity for him.
There's literal hordes of cherubic-esque Angels, only really known about by the higher ups such as the Demon Lords, that come down from the heavens and go around obliterating any city that develops too much technologically or society-wise every five hundred years or so. Some God or other equivalent of this setting has quietly forced a Medieval Stasis of the world's advancement that is punishable by total genocide and collapse of entire civilizations without warning. And no one knows why, not even those trying to keep tabs on it all.
. A woman and her child attempt to run to safety, but in a flash of flames the woman is gone. The child is left crying out in terror for her mother... before a hand reaches out to her from a distance. The fire grows intense and encircles the child, lifting her off the ground and incinerating her.