Something is ever only an interference because it is a variable.
Once Negary had a grasp of a certain variable, then that variable could no longer interfere with Negary exercising his Authority.
「 A fake will always be fake; it shall never defeat me 」Shennai's body was now fully submerged in the swamp of impurity. His power of impurity originated from inside this swamp of impurity, and the more worshippers of impurity were poisoned by his ideals to become spirits of impurity, the stronger his power would become.
Negary learnt the power of impurity from him, yet he wasn't assimilated. This meant that Negary had created a new type of impurity, which once absorbed, would allow his worship of impurity to grow stronger, capable of poisoning the minds of even more creatures.
In the beginning, Shennai only fought Negary to dispel the Path of Error and earn his contributions to the war, but now this had become a personal fight to strengthen his power.