It is difficult to believe that Ardyn was ever a true hero, even in his glory days. While it is a sure thing that he was seen as a savior at the time, curing people of the new Starscourge by taking it within himself, everything about him has always been theatrics and grandeur. In the artworks of him riding on his black chocobo down the streets, his body language screams that he is eating up all the praise, that this is what he lives and thrives on. The way he talks about the past is dripping with the venomous green of jealousy, and all of his revenge motives are rooted in it.
He believed that he alone deserved to ascend for his deeds and sacrifices, and he plays the part of the victim who got chased out. The problem, however, is that he never made sacrifices. Everything he did was for his own gain. He wanted the glory, wanted to be worshipped. His goal had never been the salvation of humanity, it was godhood. Even had he not been tainted with Starscourge, he could never have ascended as the King of Light. The Crystal lacked the power to purge the Starscourge for 2000 years, so it begs the question, how could it ever have been Ardyn?
The answer is simple: Ardyn was never the King of Light. His words are lies twisted by his own self-importance and greed. He claims to have been pushed aside and hidden away by a jealous Izunia, by one “not yet Chosen by the Crystal.” However, it is clear that he himself had never been Chosen, else things would have been far different. He appointed himself to that role because he felt he was the only one who deserved it, and by holding himself in too high of esteem, he was set up for his fall.
He had always been selfish, and when he didn’t get what he felt was owed to him, he set out to destroy it in its entirety. Moreover, he plans his revenge in a way to lift himself up ever further, even beyond his coveted godhood. Had his revenge been born of grief and rage at being denied ascension after his sacrifices, his methods would have been more simple. He could have easily just killed off all of his kin. The Astrals slept, and so had he killed all of the Lucis Caelum and Nox Fleuret bloodline, he would have won.
That wasn’t what he wanted. He was after the recognition, to be seen as a god. Because he was denied ascension as one of Light, he chose to become one of Darkness. That is why he sends Noctis on the path to reach the Crystal, while making sure he suffers every step of the way. He wants Noctis to become the most powerful that he can and to have no reason to hold back, so that he may tear apart the Astrals’ Chosen King in all of his glory, all to prove himself better.
This is made clear as Noctis defeats him, and the only thing Ardyn is concerned with is if he’ll be erased from history again.
Everything that Ardyn says and does is very deliberately engineered. He twists and manipulates with words and magic to further his goals while painting himself as victim. But he is the one who chose to spread the Starscourge. He chose to take control of his darkness, he chose to terrorize and kill much of humanity to reach fame however he could.
He used the Empire as breeding grounds for his weapons, never once caring about the casualties left in his wake. His goals were always the only thing that mattered to him. Were he truly a selfless hero, if he was ever the victim he claims to be, he never would have done all that he did. He would have continued using his magic to heal people even though it meant taking in more of the darkness that cost him his ascension. He would never have given the Empire access to the Magitek technology. He would never have orchestrated everything to make Noctis suffer in his journey.
The taint of the Starscourge cannot be the primary cause of his character. The Starscourge creates rage, and Ardyn’s anger is far too carefully controlled. He has his wits perfectly in order. He has not been turned by the Darkness, he controls it. He always has, and that is why he has never been worthy.