Per wiki policy, Spoilers Off applies here and all spoilers are unmarked. You Have Been Warned.
- The demons are legitimately terrifying creatures. They are not of that sort of Ax-Crazy insanity commonly seen in a stereotypical anime demon, but more like that sort where sociopathy is taken to the level of being a mental illness. There are at least three demons (all highly powerful ones at that) who tried to learn how to empathize with other people, but they fail to even grasp the basic concept of it and it doesn't stop them from annihilating entire cities, killing thousands, eradicating mankind and lying to catch people off guard just to kill them. It's as if there' an entire race of sociopathic, mentally unstable killers and criminals out on the loose.
- In the past, there was a demon girl who was about to be killed by Frieren's old adventuring party, only for her to be taken by a village chief in hopes of redeeming her. The village chief took the demon girl under his care, and she seems to be capable of acting like a normal person... only for her to burn down the chief’s house and kill him. Then she tried bargaining with a woman whose daughter she killed, and tried to “gift” her the village chief's orphaned daughter as “reparations”, because to demons, humans are interchangeable and one human girl should work as a replacement for another she killed. It's an extremely unnerving scene, made even worse by how the demon girl was completely and utterly nonchalant about it.
- Aura's death was unsettling. Her strongest spell falls flat in front of Frieren (who was holding back on showing that her mana is much stronger than Aura's, to the demoness' surprise) and because her spell allows a stronger opponent to take control of her, she's ordered by Frieren to decapitate herself. It's obscured by a Gory Discretion Shot in the Manga, but in the anime, her dying whimpers and the split second where you can see her cutting her neck off with red blood sloshing out, accompanied by the sloshing and "CLANG!" sound effects at the end, makes the scene look more graphic. What a horrific way to die.
- Even though Frieren takes her down relatively quickly, think about how horrifyingly unstoppable Aura must have been to the average person. Unless you've got the mana of a highly-trained mage, as soon as she brings out her Scales of Obedience, you're forced to be her slave forever. The only way someone was even able to slightly resist it in the past was through an abundance of Heroic Willpower...and to make sure that never happened again, Aura started decapitating all of her victims and controlling their corpses. So she certainly got what she deserved at least.
- The worst part is that at this point Demons are established to be The Sociopath incarnate, and also seemingly The Stoic until things go horribly wrong for them. Aura goes from an almost bored confidence in her absolute victory to shock, terror, and crying and whimpering at her own imminent death. Even the demons fear death in the right circumstances, and it's a startlingly realistic reaction for such a horrible foe.
- Macht casually burning down entire villages and manipulating two children to kill each other to see if he can feel any sort of pity.
- Solitär isn't the most intimidating-looking demon ever. She looks like a nondescript human woman with two small horns and sharper teeth, and nobody has heard about her enough to give her a title. Which is what also makes her so terrifying — she likely killed every person who ran into her just so she could be left alone. Not only that, she's also a surprisingly dangerous foe that could have killed off Frieren if Stark didn't give her a shoulder wound before and if Fern didn't get released from Macht's spell to deal the finishing blow in time.
- While all demons are sociopaths, Tob, the "Saint of the End" takes this to extremes. She reveals that she has cast a curse that is heavily implied to literally end the world in another century and she makes it clear that the survival of the demon race is nothing to her, suggesting that this will kill her and her fellow demonic brethren too. She says all of this with a straight face as if it was casual banter and invokes no Even Evil Has Standards moments from her comrades, with Solitär in particular even telling Grausam that she empathized with her.
- The further things go on as Demons become more and more of a threat, the more Frieren becomes eerie in her own right. A lackadaisical elf that would prefer to be finding new experiences or lazing about, who suddenly becomes a stone-cold Demon slaying mage that is one of the most powerful beings in the entire world when its time to bring out her A-game. If she's given a direct opportunity to kill a Demon, she doesn't even hesitate since she knows they're Always Chaotic Evil from personal experience, so there's no point in giving it a second thought. The sheer Mood Whiplash of her comedic self contrasting her Demon killer mode of work is always jarring, and she's left Demons like Aura in utter terror with the fates she gave them. In some ways, though the story fully justifies why she does it, she's about as stoic and murderously concentrated on ending a target as the Demons she fights. The way she kills them is of special note, she has no issue with ending them painlessly, but only if they accept her execution of them, and no Demon is likely to do that. She can get very visceral in her methods, exemplified with Draht, who she cut both arms off before killing him mid-sentence with what could be described as looking like a magic-enhanced stranglehold that popped his neck like a grape. All without changing her expression.
- During the First Class Mage contest, the second exam is a dungeon dive with the primary obstacle being an Artifact of Doom that generates soulless clones of invaders to protect itself — all of the invaders, including Frieren and even the exam proctor. And these clones have 1:1 copies of their movesets and are just as powerful. That's right, you don't have to imagine the above nightmare scenario of Frieren as a sociopathic archmagus; her clone is exactly that and is just as dangerous as advertised. The real Frieren spells out in no uncertain terms that if anyone except herself and Fern at the same time picks a serious fight with it, they will die even with the exam's safety mechanisms. More specifically, she states that while attacking as a large group would likely ensure their victory, the Frieren clone would kill most of them in the process. The ensuing battle is ridiculously destructive, the clone creating magical lightning that floods the whole room, what looks like a miniature black hole, and after being fatally wounded and disarmed launched a Gravity Screw that nearly crushes Fern like a pancake and breaks her staff (a move Frieren is noted to have last used on the Demon King and is so different from other magic that Fern has no idea what it even was, only realizing the feat must be "the height of magic").
