Since the fight happened inside the Supernatural territory, or at least beyond the Great Barricade wall, the Supernaturals have it free to set up anything they wanted to take down the supposed threats that all Supernatural races agreed on.
In their eyes, Rex shifted from a real nuisance into a real threat.
Not only does he gain Dual King Mark which is a feat that none of the Werewolves aside from him can copy, but he also doesn't know anything about ancient times which makes him ignorant of the real danger of the world for the Supernatural.
Every single Supernatural race has agreed to take him out of the picture.
Without a doubt, they won't let Rex's ignorance pull them down into the abyss and crumbles the entire Supernatural that they have fought for more than a dozen years now, a feat that none of them were able to do back during their time before being sealed.
Standing up to the humans is a fairytale for them, and that fairytale has come true.
