Flaws

Flaws surrounding the events of Gaia's death. She had a reason to live and fight, and it was taken from her on grounds that break all manner of not only canon, but actual science, history, and philosophy. Done solely in malice, and, by the actual words of them that did it, for cathartic effect for themselves. What follows is my breakdown of all the problems with the situation in question. If anything is factually wrong, I'll be glad to debate.

• Prop Dysfunction: She had a neck shackle as seen on page 27 and again on 76. But on 77, it happens to disappear. This shackle, as we see on the first page, goes clearly to her mandible then down to her clavicles. Getting above the shackle would result in trying to slice through solid bone (see below) and possibly part of her jaw. Under the shackle would result in thicker vertebrae and thus harder to cut.

• Time Frame. How could Scyde, in the span of a few seconds, position her blade exactly above the neck shackle? As well as position the blade so that she cut cartilage instead of bone? Even if she knows dragon anatomy by killing them for a living, there's still a positioning issue with the time given.
  

• Physics and History: The motion Scyde made would not have severed the head. Maybe a deep wound, but not a complete decapitation. In the classic sense of decapitation, the blade or other bladed instrument, usually a crude axe that was more or less just sharpened metal on a stick, would be swung a full arms length above the head of the executioner then onto the unprotected neck of the soon-to-be executed, letting gravity do most of the work. This method didn't so much cut as it did crush the muscles and bones. Later in history, when the guillotine was invented, the blade was wedged and weighted to create a clean cut. Once again, letting gravity do most of the work in a vertical motion. In the sense of Gaia's decapitation, it was a horizontal, no effort, slice. Which, in a literal outlook, is impossible.

• Biology and Physics: Scyde may be stronger than most everyone else, save for Shie and a few other "Big Cats" of her species, but Gaia, being a dragon, and thus reptilian, has hardened scale instead of human skin. Gaia is specifically an earth dragon, where her scale, along with the rest of her clan, may be tougher than the others', being built tough enough to deal with whatever nature can throw at them. Her scale would drastically reduce Scyde's effective strength. At that close of a range, there isn't enough room to get a full swing of a blade in, so therefore the swing would have almost no cutting power.
• Metallurgy: Scyde's blade is likely made of pure Zhirite, or made of an alloy of it. All that does is make the opponent dragon unable to use her magic. In no way, shape, or form does it ever lesson the durability of a dragon's scales. It may make the blade sharper or more durable in general, but not against the tougher material that is the bio-armor of dragonkind. When used properly, it's probably a dangerous weapon, but not at that range.
• Anatomy: Neck: Not only is her scales already much tougher than human skin, but then we have to take into account internal anatomy, taking liberties in assumption it's just like a human's, like their private areas.

A. The main point: Spinal column. Comprised primarily of irregularly shaped bone, that if not hit correctly, will dampen the blow of any object. In this sense, added to the resistance of earth dragon anatomy, and dragon anatomy in general, would cause almost no damage. One single slice would not cause the damage we see, though may cause a serious laceration.

B. Trachea and Esophagus. Two muscle organ systems that would also make one slice virtually impossible. The walls of the trachea are made up of cartilaginous rings which protect and support. Whereas the esophagus is a tube of thick muscular tissue consisting of multiple layers. These two structures, as seen above, form a tough, bony, muscular system that would not be damaged by a single slice, especially after being slowed or stopped by the vertebral column.

C. Musculature System. A normal human muscle system has enough power to resist a good swing with a sword. Most decapitations happen by a downward strike with a extremely sharp sword, such as a katana, or the aforementioned headsman's axe. The muscles in a female dragon would be a lot stronger, due to having to hold up her head while flying at a great velocity very often. There is a very large muscle system in the human neck and probably more so in a dragon's anatomy.

• Anatomy: Other: How could Scyde have gotten behind her to begin with? Take into account wings. Gaia's wings were directly behind her, bound together. Thus, unless Scyde was cutting at a strange angle, there is literally no way to get behind her. If Scyde did cut the bindings to get behind, why didn't Gaia just fly away?
• Morality: Here's a big one. In my opinion, what happens in real life should not affect what happens in something fictional for the public. Also, by extensive research, I discovered that she was not dead after her head was removed. The brain lives until the electrical impulses stop firing, which in some cases is up to 15 minutes. So, in that case, when her head was used as a projectile, she was still in pain, all after the agony of having her head almost literally torn off.
• Multi-flaw (Prop Dysfunction, Time Frame, and Anatomy): Gaia was leaning against a tree in panel one of 76, so apparently this is a magic disappearing tree or Scyde dragged her out a ways, propped her back up, pushed away/ broke Gaia's wings and then entered the death stance. All of which would have required a long time to do, would be exceedingly difficult to do in the heat of battle, breaks the physics of bone structure, and removes a vital prop. More evidence to show this as a grudge kill.
• Tiger Culture: Earth dragons are legendary beings to the tiger culture, meaning that they've been seen only in their history or myth. Using our myths as a reference, anyone who saw a member of the Sidhe, for example, wouldn't just blindly try to kill it, much less take a trophy from the kill. In short, what Scyde did was against their culture. If she followed the culture right, she would've taken Gaia prisoner back the camps for study by the historians. Or if killing her was beyond avoidance, Scyde would've taken a piece back to prove that she just killed a legend... and probably scolded by the elders and historians. 
• Writing with Ethics: Her story is unnecessarily tragic. Why have her center stage just to traumatize the audience later? Exile would have worked just as well. Send her out, have her disappear. Leave it ambiguous what happened to her. Not torture your audience, no matter how bad the real life events were.
• Relationships: What she did with Ghislain wasn't an affair, on her side. By the very definition of Affair, she would have to be in a relationship first, and there's no evidence of that. He was, with Oscura. What she did was a means to an end. She wanted a daughter, and since she couldn't reach the storage for the artificial birthing fluids, she found a natural source. It was akin to us turning to a stream instead of a vending machine for water. Both are water, one's unprocessed.
• Religion and Abilities Given: Where gods can be called on at a whim to rip the magic out of people, even rival priestesses as seen when Schaddig used the call on Meta, why can't Gaia's be asked to revive her? We know she's at least somewhat a devotee, as she shares a facial tattoo in her more formal appearances with the goddess. Bring her back, get her daughter out of the palace, and send her off to the horizon never to be heard from again. All within the powers of a god.
• Biological Philosophy: She's tried to run from the groups twice. Once from Elektra's group, and again from the battle. Shows her will to survive, yet when caught by Scyde, she doesn't even twitch in a struggle for her life. In the philosophical sense of Conatus, more modernly called the Survival Instinct, is the innate will of all living organisms to continue living. Beyond philosophy, it's hard wired into the brain to struggle to survive. Your sympathetic nervous system kicks in, releasing neurotransmitters to fight. Even things small as bacteria do it.