Illidan had to kill the Naaru to prove a hard message to Velen and Turalyon:
That we all choose our own destiny. Velen failed his people out of his blind faith in the Light. Turalyon was following the same path. As long as Xe’ra held sway over them they would be nothing but blind fools leading their people into disaster after disaster.
Illidan had already seen first hand how Malfurions blind faith in himself was not winning the war against the Legion, and his people were paying for it time and time again. This time the entire world was at stake. Harsh times call for harsh measures.
You saw how easily Illidan stopped Turalyon’s attack after he easily dispatched Xe’ra. This was narrative alluding to the adage that his action, as opposed to blindly following a prophecy, is the only thing that will save the world. It’s a literary tool, probably the last time a true literary tool was used in any Blizzard writing sadly.
If there was an overall point, it was not that fel and void are destructive, but that all aspects, unbalanced, are dangerous if used by the wrong people or in the wrong way, be it Light, Void, or Fel. Thus far of all the Cosmically charged leaders Illidan has been the only one to use his for good.
Turalyon did not “relent”. He was exposed by Illidan as being powerless to stop him. He took his best swing at Illidan and Illidan, even in his weakened state, stopped his attack with a single hand.
We have no character development on Turalyon after this event. We do not know how he has dealt with Xe’ra’s death. At best he has been a bit player since Legion with lip service paid to his position since then. We have no idea how he would react to the Army of Light emerging full force into Azeroth. Reuniting with Tyriel, who we also know very little about between her last appearance and today, could bring about any range of emotions with him regarding the Light and the Void.
There are simply too many unknowns to be able to rationalize Turalyon and gauge his reaction should the Army of Light come to Azeroth, or how he will react to any perceived Void invasion of Azeroth should the Void Lords appear.