Which reasoning and which motivations? When I got to the end, all I knew was that, that particular person was apparently the villain, and that there was some story there that simply...wasn't there.
Then you didn't pay attention. Ardyn was originally the Chosen King he had the power to heal those that were inflicted with the Starsourge by absorbing the daemons into himself. He did this and saved the world but because he did this the Astrals and the Crystal said he wasn't pure enough to become king OR to be able to go to the afterlife. Someone else took over the throne and made the people hate him and removed him from history.
He wanted to:
a) Destroy the Crystal
b) Get revenge on the Astrals
c) Destroy the Line of Lucis
The empire "killed" some of the astrals, the crystal was destroyed after Noctis absorbed all of the power into the ring of the Lucii, the line of Lucis was destroyed because Noctis had to sacrifice himself. The cherry on top was Ardyn was finally able to go to the afterlife.
This was explained in Chapter 13 but also when you get to the Citadel there is a line about Ardyn belonging in one of the paintings and him becoming the darkness from the prophecy.