Whille that does happens to me quiet a lot, in this case i'm basing myself on pure facts... The fact that Froustmourne consumed his soul, the fact that Ner'Zull choosed Arthas even before he was born, the fact that Arthas the DK wasn't the same guy as Arthas the Paladin... Arthas the DK had nor personality at all... He was soulness, and bound to the LK. Offcourse, he hended developing a new personality, a kind of one that allowed him to make his own decisitions without being bound to Ner'Zull... The fact is that new personality wasn't Arthas anymore... This leaded to the destruction of Ner'Zull.
But even after Ner'Zull was destroyed, yet, a new Arthas was born, he his not the soulness Arthas or Arthas the Paladin, he his a mix of the soulness Arthas and Ner'Zull. And that his the Arthas we see during all Wrath.
At the end, we can still see a smal glintch of Arthas the Paladin, ''... I see only Darkness for me'', with this we see that he feals, again, fear, and pity... And all the feelings he had before Froustmourne consumed his soul.
There is a speculation thread around, you might desire to post this there.
Seeing Arthas back is not impossible, after all he lies in front of the LK (Bolvar) which is the master of the scourge... If Bolvar wishes to, he can ressurect him, but if that happens i'm afraid to say that we might see Arthas the Paladin, not the soulness Arthas.