Basically, the Ancient Civilization did freak experiments with dragons and the dragons could no longer take losing their kind. In fact, you know all of those Frontier-exclusive monsters and Elder Dragons? Believe it or not, most of them, if not all of them, are in fact the resulting work of the Ancient Civilization's experiments. Hence the similarity between them and other Elder Dragons, like Garuba Daora, Rukidiora and Rebidiora are to Kushala Daora.
As mentioned, the Fatalis were the shining examples of the dragon's stand against them. The G-rank Crimson Fatalis was one of them. And according to some in-game info, the Fatalis used to be as many as the Rathalos and Rathian species, but after the war, they now very rarely appear. The Fatalis are also the only known Elder Dragons that all living Monsters fear, including giant Elder Dragons like Lao-Shan Lung and the Fatalis are also known to be able to burn whole areas in just seconds.
In addition, they take back their prey/victims back to their lairs before proceeding to take apart their armors and melt the armors onto their skins for their hard scales and they also do this to soldiers and Hunters, due to their enmity towards humans. Because of their rarity, the Fatalis are often compared and treated as fairy tale Dragons and are often seen with disbelief. As for the Crimson Fatalis, they are apparently Fatalis who's rage is so great that their skin changes red/deep crimson.
Here's the deal with the G-Rank Crimson Fatalis, this specific Fatalis. The G-Rank Fatalis is a Fatalis that absolutely despises humanity, to the point where it actually does willingly hunt them. The G-rank Crimson Fatalis usually lies dormant for many years, but when it awakens, it then goes on a rampage and hunts and destroys all it encounters. It is also able to manipulate lava as it's own personal weapon and is so threatening to other monsters that a simple flap from it's wings make them run in absolute fear.
In general comparisons to the normal Fatalis, because of this particular Fatalis' hatred towards humans, it not only hunts humans willingly, but also burns their carcasses onto itself compared to other Fatalis, who don't hunt humans willingy. Not only that, even it's own brethren, the other Fatalis, fear the G-Rank Crimson Fatalis. And compared to the normal Fatalis which is often seen as a myth, the G-Rank Fatalis is actually seen as very real. And that's not even all of it.
The G-Rank Crimson Fatalis is actually confirmed to be the one that destroyed Castle Schrade, the ruined place where you fight Fatalis. And to top it all off, it is not only rumored to be the Incarnation of the Original Fatalis in the lore story, but it is apparently trying to start another war. And it has been officially declared to be Monster Hunter's Evilest Elder Dragon.