By that point he wasn’t even Neltharion anymore, he hadn’t been for thousands of years.
Arthas may have had a flawed psyche but from what we’ve heard about him, Neltharion was noble and one of Alextstraza’s greatest friends. The dragons trusted so much that they gave their essence to create the Dragon Soul. He was chosen not because of a flawed psyche but because he burrowed deep into the earth which made him susceptible to the old gods based solely on his proximity to their prisons. Assuming he once more because Neltharion in death and free from the corruption, he could have a path to redemption.
Tyrande?