As for Nathanos, tremendous backstory, poor execution as a character.
Dunno if anybody’s played Elder Scrolls Online but Abnur Tharn comes to mind. Abnur is rude, self-righteous, self-absorbed, and singularly focused on his goals to the point that he will betray his allies for his own ends.
However, Abnur is also determined, brave, willing to go to extraordinary lengths to fix his own mistakes, and while he talks down to the player constantly, it’s clear that he fundamentally means well. He cares about his nation, he will protect and innocent, and even when he is acting dubiously he’s doing what, pragmatically speaking, is the lesser of two evils in any given situation.
The point is, Abnur Tharn has redeemable qualities.
Nathanos, on the other hand, is rude and self-righteous, just like Abnur. But does he have redeemable qualities? Not really. He’s selfish. He doesn’t care about the Horde. He won’t try to make amends for his mistakes, or protect the innocent. He doesn’t even really care about the Forsaken. He cares about Sylvanas. Everything that’s been written about Nathanos indicates that his personal love for Sylvanas, in life and death, was his only driving motivation. He’d follow her to whatever end, no matter how selfish and dark, even when it makes literally no sense to him.
A rude and self-righteous character can be done very well. But they need to have some redeemable qualities - things that make you know that the rudeness is a personality feature, not their whole reason for being. Nathanos has nothing to balance it against, so he’s just a jerk who likes nobody, and thus, who nobody likes.
I still think a better end for Nathanos would’ve been to go through some sort of massive character development - to BETRAY his Dark Lady, because he knew what she was doing was wrong. Perhaps he would think Teldrassil was too far, perhaps when Sylvanas left the Horde, I don’t know. But if Nathanos ultimately cared more for the Horde or Forsaken than Sylvanas, it could’ve made some great character conflict and totally changed how we see him. But alas, a devoted stooge he remained.