Aeternus is a demon. All of Legion are selfish but Aeternus embodies the ideology of taking whatever they want and destroying whatever they can't have. So if they can't have the world fragment, they might as well use it as kindling to kickstart their world engine. They don't need the first sage's agreement to do that.
The tree father was skeptical about it. He expressed his skepticism, "But it will only work once even if we manage to get it working. We will need several attempts to get our law matrix working perfectly since there might be several problems in the future. A single-use solution is not good enough."
Legion-1 also expressed his thoughts about it. "Plus it will be very dangerous to do it. The first sage will surely not be happy if we tamper with his supreme law. His unhappiness might bring problems for us if he manages to find my location. That is a possibility since we will be using his world fragment within my inner world."
