Seeing a throne he already knew about can’t be the real reason he decided to go against everyone and break all the world’s taboos. It’s way more likely he stumbled upon some hidden truth and realized he could turn the world upside down for his own gain.
The admiral’s death was probably just collateral damage. After killing him, Xebec fled. If the admiral tried to stop him, it means Xebec had already done some serious damage.
I don’t think killing a random admiral was his goal.
Anyway, we know admirals are stationed in the Holy Land during Reverie to handle any issues. We’ve seen that with Fujitora and Ryokugyu.