It started with his hatred of the Supernaturals, especially the Werewolves.
Rex has been an entity that keeps on searching for ways to become stronger and eventually stumbles across the system where the real progress has finally started, he had become very strong to the point of being able to exact revenge on his birth parents' murderers.
But his journey didn't stop there, the world is in need of help and he has the power to do so.
Even if he doesn't want to help mold the world into a better place, the power that he has would not allow him to do that. Rex can try, but he knows that eventually his power would be sought by others and forces him into doing something he doesn't want to.
Just like the tragedy that happened to his foster parents, and Kyran's kidnap.