So, I ran into some speculation about the destiny of the Lucis Caelum being more punishment than blessing and... that actually seems like a pretty compelling idea.
For one thing, Bahamut giving characters powers so they can redeem themselves is a known dynamic. The player Glaives in Comrades were all revived and granted power by Bahamut as a means by which they would be forced to atone for their betrayal.
For another, I don't remember seeing the Ring of the Lucii in Episode Ardyn Prologue. Ardyn doesn't have it, Somnus doesn't have it, and it doesn't look like Aera had it to give to either of them. It's just not there.
Here's what I'm thinking:
Somnus and Ardyn are both given powers by Bahamut and told to use them to deal with the Scourge. These powers are meant to test them to determine who is more worthy to use the Crystal to eliminate the Scourge. Ardyn is compassionate and self-sacrificial; Somnus is aggressive and efficient.
If Bahamut was looking for someone like Noct to give his life for his people, Ardyn is the obvious choice. The problem is, Ardyn is infected and Somnus doesn't work well as a backup plan because his heart is clearly impure.
Somnus, figuring that the Astrals have no choice in the matter thanks to Ardyn's corruption, moves to remove Ardyn from the equation entirely. But, unless Bahamut told Aera that he was to be king in Ardyn's stead (which didn't seem to have happened, giving Aera's shock when he declared himself king), doing so would have been a massive act of presumption on his part, which ought to have been punished severely by the Astrals.
I initially figured that the lack of consequences must have meant that the Astrals were okay with Somnus' plan. But what if the consequences were simply different from what we expected? Perhaps Bahamut allowed Somnus to ascend the throne not because Somnus made the right choice, but because it was the most effective way to make House Caelum atone for its sins.
In this interpretation, the Ring of the Lucii would be as much a curse as it is a blessing. Its downsides wouldn't simply exist because of the amount of power it commands, but because it's meant to force House Caelum to learn the virtue of self-sacrifice. Somnus and twelve of his descendants will be required to keep watch until the Chosen King, a true innocent, appears and makes the sacrifice that Ardyn was meant to make two millennia before.
That would actually be entirely in-character for Bahamut. And, if Ardyn found out about it, it could give him even more reason to try to sabotage the whole thing.
EDIT: One more thought: if the point is that Somnus doesn't give a fig what the Astrals think, that could also explain his villain act. The point of his arrogant demeanor would be to demonstrate his flagrant disregard for the Astrals and show the people that it really didn't matter what decision they made. Of course, Bahamut is an active force in the world, so thumbing one's nose at him might not be the best idea...