It's honestly bewildering to me.
Going into this expac, I had some basic assumptions about how Hydaelyn's "arc" would play out. Since it was clear--based on the fan reaction to the Ancients--that making them completely complicit in their own undoing was a no go, my basic idea of what would be Venat's motivation was this:
1. There was something more deeply wrong with the Final Days than Zodiark addressed. Whether this was something more intrinsically wrong with the planet that He couldn't fix, or something had awoken from within the depths that needed to be addressed. Either way, it's a barbed wire and duct tape solution instead of dealing with the source of the problem.
2.Venat figured out what this issue was and brought it up the Convocation. However, at this point they had all been tempered to some degree. I even remember talking with friends about how horrifying it would be to be Venat, to go to your highest authority that you're used to being reasonable and transparent and they just completely shut you down. In a world where open debate is lauded, there can suddenly be no debate where Zodiark is concerned.
3. Desperate, Venat comes up with a plan to weaken whatever caused the Final Days through Sundering. Maybe she knew what would happen to the remaining Ancients and maybe she didn't, but ultimately she knows that even with Zodiark there, the entire planet is doomed unless she does something to make sure the Final Days can't happen again.
So then, it becomes almost very amusing to me how it ended up literally the opposite of what I assumed. In trying to make both sides sympathetic, Venat comes out wishy-washy at best or nearly psychotic at worst. Instead of the Ancients being unwilling to deal with a solution to the Final Days--they have no clue! Instead of Venat trying to work with the Convocation and being rebuffed, she doesn't even bother to try! Instead of the Sundering being a necessary evil (or mistake) to preserve life, Venat decides that she is the final judge, jury, and executioner of her own people because they didn't handle the literal apocalypse with as much pluck as she would have liked.