Live Letter Thoughts

[Kazhar]

They had the intent to destroy a part of their own people for the sake of summoning Zodiark. They did so.

After they did that, they had the intent to destroy a part of their own people to repair the damage to Etheirys. They did so.

This is a disingenuous way to phrase it. It's been made very clear that the Ancients sacrificed themselves to summon Zodiark and that they did so willingly, the story never supports that they were forced nor brainwashed into it. Whether the Convocation could be still considered responsible for their sacrifice is an interesting question, but it's certainly not as clear cut as the way you say it.
Only the third sacrifice was clearly framed as killing other life forms without their consent, and this is not a coincidence that this is the one which sparked heavy debates among the Ancients society and forced Elidibus to come back as an Emissary.

So why do people want to venerate the group that succeeded in 2 acts of genocide and were about to commit multiple genocides while demonizing the individual that stopped that massive wave of genocide

I believe people (those of us who were disgusted by Venat's act) would have a much different reaction to the Sundering if it was established that Ancients agreed to it. All of this was done without their consent and a decision taken by (possibly, depending on how much she told her followers, and we know she wasn't really found of sharing) a single individual. The post EW live letter supports this interpretation.
So yes, this is not comparable.

And even if we accepted your notion that the willing sacrifice of the Ancients was an act of evil, you can't absolve Venat of responsability for it. After all, her entire 12k years plan needed Zodiark to work in the first place.