Ardyn was the first Savior chosen to save the world from the Scourge that Ifrit called upon people. But Ardyn's body was different, he didn't just heal the people, he absorbed also that darkness into his body. Although he didn't mean anything evil, that absorbed power made him strong and also immortal. But the God's got scared of him, because he got even stronger than themselves. So they decided to cast him away, and chose another Person in his place, that was his brother Izunia. They also split the roles of Ardyn, into the Oracle and King, with this also the line of Kings started, and the line of Oracles.
As you can see, Ardyn was beloved by everyone, because he healed the people, and saved the them from darkness.
He is kinda display as Jesus, and at the end he was betrayed by his own kind. That's why he is seeking revenge, that's why he want's to end his own line, and that's why he want's to get rid of the Gods.