As Cruze spoke at length, Xiao Ran came to understand the experiences Cruze and Graham had gone through in Gundam W World.
Simply put, after learning about the situation in Gundam W World, Cruze saw that the Earth Federation's rule and the Space Colony Alliance's resistance were almost identical to the situation in his own world, Gundam SEED. Rather than truly joining either side, he chose another path—the one Lacus, Kira, and the others had chosen in the SEED World: peace.
After careful analysis, Cruze discovered that although the Earth Federation was nominally one faction, it was actually divided internally into two opposing camps: the Peace Faction and OZ. This provided the foundation for the path Cruze had chosen, ensuring that Prometheus's missions would not interfere with his actions.
After joining the Peace Faction, the missions Cruze received were naturally related to the Peace Faction, rather than missions issued by OZ that would conflict with the path of peace he had chosen.
And speaking of peace, perhaps Cruze simply felt that destroying the world once—and nearly succeeding—had been enough. Having once been a demon king bent on world destruction and savored the experience, he now wanted to try being the world's savior, to experience what it felt like to be admired and worshipped by everyone in the world.
Although Xiao Ran did not know whether Cruze truly had such intentions, the final result was that Cruze really did become that world's savior.
As he gradually became one of the Peace Faction's key figures, Cruze had tricked at least a third of the participants who had chosen to join either the Earth Federation or the Satellite Alliance. Then, through a deal with a certain doctor who controlled OZ, he directly pulled out a gun and killed the man in full view