A>B(>C) logic is not very usable against foes like Lich King for one, or other characters who does not rely on a lot of damage to win their fights.
Then the questions comes: Can Diablo see across dimensions? Can he survive the Frostmourne? Can he harm the Lich King in the spirit world? Things like that.
I think lorewise Diablo is much alike to Satan from Christianity in many respects. He claims that he takes souls with his "not even death cna save you from me" and the like which means he must be able to see spirits and he is ruler over hell where dark souls go. And Diablo can see/appear in the mortal realm at will from hell.
I doubt the LK could stand the sight of Diablo, the LK or Arthas in general have a lot to fear from their past and possibly their future.
I hear the LK is more Arthas, if not entirely Arthas rather than Nerzhuel? therefore ghosts of his past such as his father as seen in the intro to the new patch would likely haunt him to distraction, maybe simply the fear of his own destruction. Either way whatever the LK fears most Diablo takes the form of it. Even his servants who were masters of fear themselves almost flee in fear at his very presence.
Diablo even at his weakest could give life to the nightmares of Prince Albrecht, a young boy who was brought to him through the use of illusions and his corrupting influence on an Arch bishop. This was all while he was nothing but a soul trapped in a soul stone, arguably the weakest Diablo has been.
I think its possible that at full power Diablo could combine the above stated to put fear and confusion into the LK enough for him to destroy himself or give Diablo the opportunity he needs to devour the soul.
He has lost his love, killed his father, banished himself and the entire world hates him.
Seems a little far fetched to turn his fears against him. This is the guy that walked around without his still beating heart. In Rise of the Lich King book, the last part of him that was the slightest bit human supposedly died.
As for the trailer, it does not imply anything at all. The only thing you see going on there is Arthas listening to his father, completely devoid of emotions.
Then a conglomeration of those fears would be arrayed in Diablos form. Fear does not disappear with humanity, theres little to claim he has no fear, nightmare or dreams. In fact being the LK his mind is likely full of many things that would be considered nightmares and could be given life by Diablo much the same as he did with albrecht.
Actually, iirc Arthus was apparently devoid of emotions. I remember in Warcraft 3 a Dreadlord asked him whether he felt anything after killing his father and he replied in a detached voice that he actually didn't feel anything. I don't pretend to be an expert though.
Which are completly different to fear, assuming he has no emotions when both Nerzhuel as the LK has shown them as did Arthas obviously as a human. Both have emotions, why should he lose them now? why should we assume he does not know fear when theres nothing to support it?
His conciousness is enormous, its highly unlikely fear, even the smallest amount would not be among such a mind. Also didnt the LK retreat from several battles? such as the forsakens gas that sent him off into his citadel, Ashbringer which pushed him back?
If for some reason he had no fear then Diablo would be invisible I guess
You think fear is required in order for someone to retreat? Anyone with the capability to think would retreat from those battles that Arthas fled from. Just because you do not fear, does not mean you just drop dead when you have the option to survive.
Then he obviously fears his own destruction and has a will to survive. Hence perhaps Diablo would take the form of his own death. Nerzhuel also sounded fearful when Arthas was on his way towards him as his life force was seeping from the throne.
Ner'Zhul was still the LK, he had fears and emotions yet obviously piecies of these characters remain otherwise why would he still have his father in spirit form? I probably would enjoy it.
Dying is stupid? tell that to all those mad mages who become liches, they seem to do well out of being dead. Arthas/LK is arguably dead in a sense....your assuming the LK perspective on his own death just as much as I am it seems....
Either way, being fearless or not, Diablo gains an advantage from either.
His father imply that all the souls claimed by Frostmourne remain within it. The fact that his father speaks might be for no other reason than because his father wants to speak. As you can see in the trailer, he is not the only soul there.
The mages did not so much die as ascend into a greater form. They traded mortality for immortality. Arthas being slain can not be compared to that. Arthas dying would be a strategical setback for him and he has had enough of those. He would probably prefere staying alive.
But then again you could be right, that might be just me jumping to conclusions.