Her decision isn't treated lightly or with reverence, it's not exactly hidden that she feels great sorrow over the pain she caused but knew it had to be done in order for mankind to be strong enough to overcome the inevitable Final Days. It's not exactly the most complex writing.
Venat isn't a god. She's a primal like all of them.
Except unlike those of the beasts who they truly believe to be a god, or Zodiark who was brought into existence as a tool to protect them, Venat most cosmologically resembles
Shinryu (technically Asura...) than any of the other primals, something brought into existence by one vengeful being to destroy the status quo.
She doesn't know that it's going to work. She just thinks it will because she thinks that we're in a closed time loop.
She ripped the souls of her very people apart on a risky bet.
They all writhed in demented agony in the forms of the Cubuses (This is why Emet-Selch put them in The Tempest.), until they finally regained human form without any of the memories of society or who they once were.
She's a slave to fate, the worst kind of unintentional villain.
It didn't have to be done. She just thought that it did. Now we have a bootstrap paradox.
There's also the fact that Hydaelyn, and the Twelve, are all deadbeat parents. They don't do anything to help their people and are just there, sitting around in the Omphalos.
The Twelve didn't even help when Dalamud fell - They say that themselves in the Myths of the Realm storyline!
But yes. Let's go simp for the deadbeat cosmic flat-earther because she called you her brave little spark in that song. It's not like we're only able to get to Ultima Thule after the events of Elpis because Venat put a tracker on Meteion that the Ancients were likely planning to deal with themselves if Venat
who wasn't affected by Kairos the Dementia Machine, had been selfless and told the others about that, allowing them all to look for where Meteion ended up and send their own Entelechies (Or dragons. Just make a Midgardsormr, considering that the reason why he grew to such a massive size and unusual shape was to absorb enough aether to survive while travelling through the sea of stars) to beat her up after they'd made the second sacrifice to Zodiark.