Saleen had no intention of leaving any bit of dignity to the Tanggulasi royal family. The reason he didn't attack was to make sure the conquest of Nature Faith lacked the final bit of legality to it.
When the four empires signed the treaty all those years ago, it was stipulated that they were to not attack each other. With the Tanggulasi royalty staying intact, it served as proof of the empire's continued existence. If the other three empires showed up or picked up anything left in the empire, Saleen would have been allowed to destroy them.
While such reasoning was of negligible use to powerful beings, it was truth to the tens of millions of common folk out there.
