I actually went and watched the entirety of Episode Ardyn because I was so unhappy about something that I got spoiled on unintentionally (i.e. Ardyn's corruption was all according to Bahamut's plan) that I wanted to make sure I wasn't going to pay for something that completely destroyed everything I liked about the game.
With that said, I think I'm okay enough about it after watching it that I don't mind paying $10 for the opportunity to collect all the extra lore that the streamer didn't go through, so...
The impression I get is that the Astrals* see the universe as fully deterministic. It's not that Bahamut came up with a cruel plan because he wanted Ardyn to spread darkness and for Noct to kill Ardyn to restore the light; Bahamut just knows how things are supposed to happen and gives them a push when necessary to make sure they go that way. Ardyn's calling is to do what he is already inclined to do, based on the consequences of his powers and his experiences, which is why it literally does not matter for canon whether Ardyn submits or resists**.
In the end, then, the Astrals are kind of like Fal'Cie, following a will greater than themselves without questioning it. House Fleuret and Somnus' branch of House Caelum accept that as well; Ardyn doesn't, but he can't actually change anything. It's pretty clear that the Dawn of the Future storyline is meant to reflect an alternate universe where Noct and Luna figure out some way to undo the determinism of the universe, but I'm not sure it's really necessary for Episode Ardyn to function within canon.
And, on the religious symbolism end of things, Ardyn kind of ends up as a weird fusion between Jesus and Judas Iscariot. Ardyn accuses Bahamut of making him a sacrificial lamb, but he's really more of a scapegoat in the final analysis. He's given the role of taking all the darkness and evil into himself so he can be cast out to restore the light. Noct, in contrast, is the innocent whose sacrifice is necessary for the light to be restored.
With all of that said, on a characterization level, I really liked what they did with Ardyn. The transition between the selfless healer and the complete sadist made far more sense than I expected it to, thanks to the combination of Verstael's intentional manipulation and the effect of the memories he absorbed when daemonifying people. It's not too hard to imagine that Ardyn's theatrical personality was the result of him having far too many lives inside him to retain his natural individuality, and that his sense of knowing the world like the back of his hand was the result of the number of people he'd absorbed rather than the amount of time he was able to roam Eos.
* Apart from presumably Ifrit. I highly suspect that the corrupted vision of Somnus and Aera after Ardyn daemonifies Ifrit was at least partially Ifrit's doing.
** The credits scene strongly suggests that the "resist your fate" ending is the canon one.
