This story is not presented as a myth, and Venat is not presented as a God. What happened is something we largely see play out in real-time before us with people we have tea with, help with mundane tasks, all while enjoying their charms and quirks and personality nuances. Much of the pathos of Venat's story is from meeting her as a person, and coming to understand that rather than a God, she was simply a single human person who made decisions on behalf of her vision of what humanity should be and her vision of the greater good for all of life and existence.
The attempted re-painting of our encounters of the Ancients, and the actions they committed, both for good and ill, as 'simple myths' or 'ancient history' always feel weird to me, because this is a video game where there is no meaningful distinction between the NPCs we talk to and accept quests in the 'present' and the NPCs we talk to and accept quests from in the 'past' (which we can access simply by clicking the teleporation button.) Either way, in our immediate moment, we talk to them, get annoyed, laugh at them, engage with them equally as pretend-living people in front of us. Even beyond one of Shadowbringers's prime writing goals being stated explicitly as "humanizing the Ascians," Endwalker itself is as firm as it can be in the final word on the Ancients through things like the DoH quests, or G'raha's remark in Ultima Thule: they were simply people.
"God" gets away with causing the great flood because he is a legitimate god, fundamentally inaccessible and incomprehensible. (And even then, opinions vary - I'm not religious myself, and the suggested nature of most 'gods' in theism is one factor as to why.) Venat is (and the rest of the Ancients, and the Ascians) explicitly a relatable human person, so suggesting we should downplay the decisions she, or the rest of them, made as relatable human people by acting otherwise is very strange and does the story a disservice in and of itself.