Arthas is fully aware after he was pruned by the soul of Ner'zhul, and he never didnt have control, thats what makes his story actually good, however that doesnt mean Ner'zhul doesn't have a hand in things given they are using his phylacterys for their power.
In the book when Arthas kills Ner'zhul in his mind he only excised that soul from his own, in effect completing his power and to silence the necromancers whispering, thay doesn't mean he excised that power from his phylacterys, doing that would make them completely inert, and would also have other consequences given his power is bound to not just frostmourne but also the crown and the frozen throne itself, destruction of the throne would have consequences for the planet and would unbind the scourge from servitude.
Yes Arthas was Arthas, however his intents were warped before he realized the monster he had become and through Ner'zhuls machinations Arthas was shown the true might of the powers he had acquired. Arthas came to the conclusion his way was right on his own, but he was pushed in that direction and because of that his hand was forced at most turns to become what he was.
Bolvar is still Bolvar, however same thing, his intents are being warped and he himself upon taking the crown and throne foresaw this, due to the scourge always needing a master and however the throne is connected to the planet there must always he a lich king, Bolvar was aware that he was making a sacrifice and tells fordring as such, this directly implies the evil of Ner'zhul is still very much present and he was likely combatting it the moment he became the new lich king, however Ner'zhul wouldn't be at full strength given he was used as the source of power Arthas just expended. Ner'zhul is more slave than master but he is still pure evil and would still attempt to assert his will however subtle.
Hes the The Dark Knights Joker to Harvey Dent, Dent made his choice of his own free will, but it doesnt mean his perceptions weren't completely warped, live long enough to see yourself become the villain, hes not an Ozymandias like Illidan "salvation by any means even througu mass death and destruction", each of the villains is just a trope, and Arthas is the embodiment of "live long enough to become the villain" but there has to be a reason for the fall from grace, first it was a dreadlord, after it was Ner'zhul who doesnt care for the legion just as much as he doesnt care for the creatures of Azeroth.