By telling Cayden that he would earn merit from "the family", the enemy leader could overwrite the personal requests of the princess that supposedly sent this "executor" to extract information from him.
The enemy leader did so, because after knowing that Vivian hated him at a personal level, it wasn't weird for the one to be sent to extract intel from him, to be told to cause him as much pain as possible, but this way that could be omitted, because everyone wants to raise in ranks in this cultivation world, so between having to put effort in following a personal request and only be nodded at by your direct superior, or finishing up your really fast and be properly recognized by the "president" the choice is obvious.
