This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / White Dark Life
The demon characters all have some horrifying aspect about them (except Priscilla). Damien can form mouths almost anywhere on his body but that's relatively tame compared to Dark Matt and Artemis. Dark Matt is in permanent pain because of how poorly his body is built, requiring stitches to keep what still exists together. But Artemis is worse. Her arms are long and boney and rather than hands, she has one replaced with bird-like talons, and the other is replaced with large boney saws. Her stomach has been completely erased, with her spine and ribs completely exposed. Her face hides behind a twisted grinning mask, the only part visible is a single bloodshot eye. Oh, and she can drain your life force on contact. AND DARK MATT MARRIES AND REPRODUCES WITH THIS THING!
Dark Matt gets a new form after the timeskip. His upper body is wrapped in thorny vines, his lower body is potentially nonexistent, and his head is obscured by his own skull.
Damien too can turn into a horrible monster. His lips disappear, giving way to a jaw that practically goes across his entire head, another mouth appears on his chest, his arms and legs are covered in tooth enamel and are VERY pointy, and his teeth become even sharper with a Venom like tongue to make things even worse. And like with his mother, a mortal (DREAM in the roleplays) reproduces with him.
When asked about it, Dream's only complaint is the possibility of getting poison gas to the face.
Eloria is a Wendigo. Yes, the mythological creature that gets stronger the more it eats. And given that Eloria can feed off of anger itself...
Luke, THE Devil of the series, is a literal Hate Sink to the point that Eloria looks tame in comparison. The Demon Lords are all outright afraid of him, seeing him as less of a person and more of a god/force of nature to keep appeased. The only time he's ever been defied was when Artemis was a complete lunatic.
Altair sees nothing wrong with the wholesale slaughter of demons. This includes children who can't defend themselves.
This only scratches the surface with Altair. His real power is absorbing souls and erasing the memory of them from the world. Imagine everyone you love forgetting you ever existed.
Blake has Damien's skill sets, but not his Weaksauce Weakness of a picky stomach when it comes to meat. This is one of the major reasons why Damien does not eat people, but the psycho with similar powers doesn't have to worry about this.
It's implied that Damien has eaten a person at least once while working under Eloria. In fact, he eats his own WIFE AND DAUGHTER without hesitation in his arrival in Easter Teeth! He did so as an impromptu way of getting them out of the way of an otherwise unavoidable attack, but still...
And before that, we have his first moment of humanitarianism with Momoko. At least, it's Nightmare Fuel for her; Damien being delusional and recently taking "precautions" might make the scene less scary, or maybe worse depending on how you view it.
Damien's body in general is creepy. He's capable of surviving his limbs being torn off and simply forms tongues on the wound to pull them back in place, he can distort the length of his body by forming screaming mouths on himself, and when he spits out Momoko, his stomach of its own volition declares Momoko inedible, making Damien literally incapable of eating her.
Timeskip Dark Matt is a freaky individual who curses sinners for their actions and rewards the good by writing in a book using his own blood. He has also formed a sadistic nature towards people he curses, going as far as in one roleplay as devouring the heart of one villain's girlfriend as payment for a wish he refused to grant. To be fair, said girlfriend was a nasty villain in her own right.
The RP from the comic revealing this paints the robots as just as murderous as their game counterparts, and even more unstoppable (except Bonnie), to the point that Eloria, an adult demon capable of destroying almost anything, is just as helpless against them as Damien is oblivious to the fact they are dangerous.
Malthus and his top subordinates, the Chi Trinity, are capable of manipulating the Lifestream. Yes, thatLifestream. It's just as dangerous as in the source material; raw Mako can poison living beings and strip them of their memories, and death from direct contact with Mako results in said Mako absorbing the victim's soul, leading to Cessation of Existence. Raw Mako also happens to be extremely effective at damaging or destroying The Undead, due to a combination of Revive Kills Zombie and being equivalent to a Holy Hand Grenade in the lineup of Elemental Powers. Oh, and did we mention that Malthus is a huge fan of Revenge by Proxy? Piss him off enough, and he will not leave you alone until everyone you love is dead.
The Trophic Blenders want to "erase the divide between plants and animals". To that end, they abducted countless Seedrians, prompting the rest to resort to... drastic measures to try to stop them. Or, to put that in more precise terms, they are directly responsible for the Metarex's Start of Darkness, and all the horrible stuff that happened to the Seedrian race and the universe as a whole as a result of that. And as for what the Trophic Blenders do with their captives? They're running People Farms. And they're abducting all sorts of animal species and forcibly cross-breeding them with the Seedrians. Yikes.
Then there's Icing the Rabbit, whose daughter, Dough, was abducted by the Trophic Blendersnote actually, she willingly went with them to get back at him for being neglectful. His response? To band together with others who have lost family members to the Trophic Blenders' antics and form a militia to massacre the children born from the Trophic Blenders' People Farms. Pay Evil unto Evil wouldn't even begin to cover it, except that the children Icing and his militia are attacking are innocent. And, yes, Icing even goes after his own granddaughter, Forest. Thankfully, Isaac, Mordred, and Cream put a stop to his antics.
Oh, and Black Doom and the Black Arms also show up. Yes, they survived the events of Shadow the Hedgehog. Thankfully, they lost the Black Comet, and with it, their endless supplies of paralytic nerve gas, but still, they're still out there, still eating people whenever and wherever they can get away with it.
Ben would like to take a minute to remind you that Black Fortress experimental projects, while capable of being Fetish Fuel, are not ever. They come with thousands of different risks. Mental instability being the foremost of these, Body Horror being another.
Miridian Violet "51" vials cause genetic mutation upon injection, starting with rotting off the very flesh of the victim, and it will mutate you into a far different form of being, and that new form is dependent upon who the vial's contents are emptied into. Side effects include development of unknown, uncontrollable powers (without training), metalification of the victim (makes you a statue for the layman), loss of memory (complete wipe, no getting it back), and more are undocumented (oh, and all of these are not in all cases). Oh, and if the transformation just so happens to fail, to quote The Doctor, "I am so sorry". It is yet to be revealed but Test Subject 11, whose name we have yet to learn, fell victim to this.
And the Trophic Blenders are actually trying to steal this stuff and use it for their own purposes.
Dark Matt at one point claims that if he really wanted to, he could curse someone into turning into Double.
The Maid blames everyone else for her misery, to the point that she will kill their souls if she gets the chance.
At times, Matt's Overprotective Dad tendencies lean towards disturbing levels, like when he starts exploiting Torch's psychological fears.
Matthew and Ne have been put into horrible situations. Specifically, Matthew was put in a repeat of the same events that killed the five children at Freddy Fazbear's Pizzeria (he survived of course, just barely, albeit incredibly scarred from the incident mentally — especially the fact that four other children diedand he was completely unable to save them), while Ne is hinted to have visited Mogeko Castle, which, given what the Mogeko are like, helps explain his sexual confusion.
Miriam really doesn't take the latter very well. It ends up less of her being Violently Protective Girlfriend and more "I will murder an entire castle just to be happy with you". Keep in mind that Ne is deathly afraid of Miriam due to the fact that she reminds him of mogeko Castle.
And then there's Viv, who wants to murder Miriam just so that Ne won't be reminded of Mogeko Castle by her anymore. (And also because Viv wants Ne for herself.)
Speaking of Mogeko Castle, King mogeko is even worse than he was in the game, being noted to rape teenage girls through every hole except the eye sockets simultaneously. He also performed a wickedly effective Batman Gambitto claim Miriam that only failed because of Matt's Blood Knight tendencies.
Dark Matt willingly revives Wrapsodi just so she can die right in front of her daughter's eyes before he starts attacking her.
Uma became so pissed off at Luigifan by the time of "Scardey fox Training" that she is willing to either sterilize or kill herself just to spite his desire to pair her up with Matthew. (Though she has threatened to do so before then.)
TORI becomes an antagonist at the end of Scardey Fox Training. Divorcing Matt (though she also reveals she never legally married him) and declaring revenge on the entire Heroes League out of pure spite against Mordred. Sure, Mordred did try to kill each one of her children over the course of the roleplays over time, but Tori makes it very clear that she will use any means necessary to hurt Mordred, to the point that her grudge will linger on even if she is killed, and will quite possibly accomplish this through possessing Ne to attack Miriam.
After this, Matt's level of violence starts spiking to disturbing degrees as a coping mechanism for his newfound depression. At first, he simply decides to stop goofing off in fights, as with Vaati, where he just flat-out tries to beat him to a bloody pulp. The next roleplay ramps it up where hekills Bowser Junior's entire army by sucking them into the void as retribution for attempting to attack Ivy, and this was after already beating them up to the point of them being considered defeated. And he doesn't even bother with making sure Bowser Jr. doesn't make off with Leila, when kidnapping or killing her was the entire point of his attack. The only thing more disturbing is that IVY JUST SEEMS TO HAVE ACCEPTED THIS, albeit, really begrudgingly. He gradually gets WORSE!
In "Scardey fox Training 2", Moge-Ko manages to escape from the now Artemis-run Mogeko Castle. Repeat. THE ONE THING NOBODY COULD STOP IN THE MOGEKO CASTLE ARC IS FREE!!
Even worse, she's well-aware that she's lost some of the factors that made her so dangerous originally (namely, an endless supply of minions and carte blanche to do whatever the hell she wants). She has compensated by becoming extremely methodical, cautious, and cunning, deliberately stalking her prey and striking at precisely the time that they're most vulnerable, while also constantly staying on the move to evade Mogemiss and anyone else who might try to stop her. So, basically, she's become a Serial Killer.