In fact, Croix had the opportunity to assassinate Isabel back then.
She had already gotten within a few meters of Isabel, and there were no "elder" Transcendents present. Aiwass's healing abilities at the time were far from sufficient to treat the fatal wounds inflicted by a Fourth Level assassin...
—A mere flick of her finger, and the course of history would be changed by her.
It could be said that the future of the entire world was thus held in the hands of a minor figure.
In such a circumstance, Croix missed that one and only chance.
It sounds unfortunate, but it was also inevitable. Because that was not her mission originally, and in her understanding, Shadow Stealth couldn't be detected, so she didn't bother to take extra precautions. Moreover, for her, killing the Royal Family was meaningless—it was a task entrusted to her mentor, "Bone Sculptor," whom she trusted completely. As an assassin on the Path of Adaptation, survival was her primary concern.
