Emotional Resonance. If a Nastika king loses control of his emotions, the lesser sura and halfs go berserk. Halfs are just about everywhere; they have super strength and possibly transcendentals. They will attempt to kill anyone in this state, even friends and family.
You know that cute little guy who's just so deeply in puppy-love with the main character it's beyond sweet (to the point he'll try giving himself food poisoning on a regular basis)? You don't want to get on his bad side. Really. You don't. You know it goes beyond Game Face when Nastika Kings start pulling Oh Crap expressions when he lets loose. And, that side of him? Is never that far away.
According to Currygom, whenever Yuta eats anything in his human form, it resembles the aliens in Parasyte.
The humans hunting sura. At least other sura just kill their opponents, not make ingredients and products out of them.
The Cataclysm was a horrific event. Not only were the Halfs going crazy due to Emotional Resonance, there was news each day about planets being destroyed.
Hoti Yama; it is an execution spell. The victim's lifespan is eaten up within 24 hours, and it affects everything within a certain radius, and sura are only immure because they are they The Ageless. Not even the caster is protected, and has to remove the spell.
In Half, a trio of slavers strike down the Half he's rescuing. Yuta's immediately enraged and thinks (recalling his promise to Shuri) he won't eat them, he'll chew them up and spit them out.
Takara's first appearance - the page image - is just a HUGE pair of red eyes in an ice water channel with a dark ceiling. Those little dots of color near the bottom? Those are the main characters. And then when Yuta moves towards her, she lifts her arm and her index finger unravels into a series of tentacles.
A couple cheerily talking about buying a Half to kill him to celebrate their engagement. By the way they talk, it sounds like they just talking about buying food for dinner. We even get to see the chained Half in question that was gonna be killed and how people around get excited to see the act.
When Agwen encounters Yuta, all she sees is a potentially hostile Half with a god-level item in a blood-spattered clearing. When she tries to peacefully restrain him, her magic doesn't work. She clearly felt she was in danger.
Yuta kills a sura while making its friends watch. He then eats them all while they were still alive and squirming.
Taraka herself is this, and also an in-universe example. All other sura, including the ones in her own clan, are afraid of her.
Teo catches a glimpse of Gandharva's sura form in the Lake of Reflection. She just found out her houseguest is a Humanoid Abomination and if she gives away that she saw, he would most likely kill her. Just think; she found a stranger injured outside of town. She let him through the checkpoint, and let him stay in her home. One day, he helps her out at work. At work, she discovers, like seeing a vampire's reflection (or lack thereof), that he's a monster. And she doesn't know what he's after.
While Teo does research on sura to try to find information on Gandharva, she comes across information that says he's destroyed at least one populated planet, and eaten a star.
Gandharva's berserk attack on Kalibloom. He completely flooded the lower half of the city when he took his sura form, which dwarfs the city. Then, he destroyed the barrier by headbutting it. It's clear that no matter how great a magician like Asha is, there is no hope of defeating him.
And this is when he's weakened!
When Agni arrives, he notes that this is the smallest he's ever seen Gandharva. That's right, the gigantic monster towering over the 9,000+ meter high Kalibloom is considered small.
Yuta behavioral changes after reaching his 3rd stage. His Lack of Empathy becomes more pronounced, and he starts expressing the desire to kill Leez.
Yuta loosing control and eating Leez's face.
Claude's proposal on how to keep Yuta on the side of humanity: to allow Yuta to repeatedly eat Leez because she's the reason he's there, and she can just regenerate anyway. Ran is understandably horrified, but no one agrees with him because of the The Needs of the Many attitude that they all have.
Asha is a Serial Killer and has been since the age of ten. She's killed over 60 people.
And then there's the way she killed them. She would use them to practice her spells. She would use Hoti Visnu to resurrect them, reset her daily spell uses, and recover her vigor. She only stopped when she ran out of Hoti Visnu uses for the day.
In the Flashback Arc of Asha's past on Carte. Carte was flooded through its many water channels. So much, actually, that it changed the topology of the planet, and killed over twenty million people. THEN there were many, many Red Sky incidents, as to kill the survivors who fled to higher ground. Those apparently killed over four hundred thousand people. This was most likely done by two beings- Maruna and Gandharva. That's right, a powerful Nastika/Rakshasa pair can ravage a whole planet in a few weeks' time, and that was when most of Gandharva's skills were sealed- Now he has access to them again.Humans can be gladhe's on their side now...
Visnu isn't as benevolent as he initially appeared. In fact, he's downright amoral and manipulative. He clearly manipulated Asha into accepting his deal, and even if she's responsible for her own actions, he clearly tailored his words to influence her.
The Takara parasites: They've been infesting and taking control of sura from other clans, and stealing their power. If one were to get ahold of a nastika like Gandharva, which is an In-Universe concern, it'd be nigh unstoppable.
When Maruna manages to reach the sura realm, he finds chaos as what appears to be sura from allied clans battle it out. When he asks Akasha what's going on, and where are the Garuda, she states she's the only one left who can fight.
During Agni and Chandra's discussion, they believe that the Takara have overrun the Sura realm and devoured almost everyone there.
Gandharva, when speaking to Maruna, implies that Kali is totally off her rocker and seeks the end of the universe. She's so bad apparently, she's been sealed over a hundred times over the history of the universe.