Rowan killed Dorian without any fanfare, as he did not believe in flashy battles when a single cut would do the job. He was not above lying to his enemies if it would make them stretch their necks forward for his blade. He learned from the lessons taught in the Nexus.
Although he was aware of the reason Dorian might believe his words as he understood his character well, and also the Empire still had nobility of a sort among its elite.
Power was respected above all, and for those that have power they should also have the dignity associated with that power. That was the reason the gods badly partook in mortal affairs.
In this world that was so entrenched in tradition, there was a set rule about which they all followed, which was not surprising given that the Empire had people in it that were functionally immortal, and changes were slow if not non-existent. Traditions tend to stick for long.