Not really a good cover, especially with Encyclopedia Eorzea 3 out.
Zodiark was made to stop the Final Days as The Ancients understood them. He required massive amounts of aether to do this, and he was successful, but the planet was still burnt up and ending as shown from the orbital view of Etheirys in the Amaurot dungeon. So they again made another sacrifice, the number of sacrifices now totaling 75% of the survivors of The Final Days. They then planned a third sacrifice that would have to wait for new life to crop up on Etheirys, their intent being to sacrifice new life to bring back their sacrificed friends and family.
Venat was resolved to see the time loop through, or a slave to fate, pick your poison. There are no other valid reads. To do this, she had to incite a civil war by taking followers onto her side. Followers who were exempted from or refused to sacrifice themselves in the first place, leading to some very sticky morality on display, since the game intends for us to view Venat as a hero (not evil at a minimum, as stated/requested by Yoshi-P).
To justify their civil war, she lays a lot of blame on Zodiark saying that the Ancient Ascians have already gone too far, given him too much, and that another sacrifice to bring back the ones first sacrificed would be wrong and against Amaurotine Ascian code of conduct. Nevermind that she eschews that code of conduct herself when she refused to return to the star after fulfilling her duty as Azem in some hundreds or thousands of unspecified years before even the events of Elpis.
To become Hydaelyn she lied to her followers and then had them all sacrifice their lives to her, and in order to make herself strong enough to perform the Sundering and contend with Zodiark, their souls and lives and all future possible lives were burnt up completely in the ritual. A ritual that they had to steal from the Convocation, because it is the same ritual as the one used to create Zodiark.
The game's narrative also has a haphazard way of displaying that Zodiark tempered the Convocation. Emet says he did, but shows no signs of tempering. None of the other Convocation members show any signs either, and even the one who became Zodiark's heart is still able to be more reasonable than any other tempered person we've been shown in the setting. We get a tongue in cheek joke from the Hummingways about Zodiark being so powerful that his powerful seduced their minds, "a little tug." This makes it different from other tempering, which is chocked up to being a purposeful alteration and ability granted to primals by Ascians altering the creation magicks for them.
This is all mildly at odds with the only actual in game portrayal of the Amaurotine civil war that we're shown in game, the Qitana Ravel cave paintings, which clearly depict a being of white and a being of black clashing with followers in the shape of men beneath them of matching colors on opposing sides.
So Zodiark is a misinformed, literal savior god who may have been able to solve the problem with the right information. Hydaelyn is a god made to split him, and then kill all of mankind through a reductive, unstable mutation. This ensures that the Warrior of Light and our present would come into being, but it is very evil on an absolute moral level.
To accept it as the only way forward clashes with Final Fantasy's most basic premise in the series' very first entry. It also means you must accept that in spite of having full control over The Echo, and having an unremoveable tracker branded onto Meteion, there was no way to tell the Convocation and make them believe Venat. Which simply isn't true, give that The Echo lets you open the whole of your life's memories to other people (see Fordola experiencing the WoL's entire life in Stormblood for an example), and if Venat were to do that for Hythlodaeus and Emet-selch, they would regain the information lost in their memories from Ktsis Hyperboreia.
Never mind the kick in the teeth at the very end of everything as Meteion is dying, "What I was looking for was on Etheirys all along..." Literal in game confirmation that there was nothing actually wrong with the Ancients, and that it was merely Venat's fear and co-opted power that was their undoing.
She could have been a real hero. She could have been their savior, even. Instead, she's responsible for more death in the setting than literally anyone else, responsible for more than even Meteion.