I feel that people invent a sentient race never seen or heard of before or after the Sundering to justify Venat's actions because the truth is so horrific, headcannon has to makeup people for her to save to live with it. It's the same way people say that the Ancients deserve to die because they didn't respect life, they were willing to turn a butterfly into a robe to my level 90 leatherworker...
Every excuse we ever give to justify the eradication of these people, the sundered races do the same but worse. So in the case of the third sacrifice people tell themselves that the Ancients wanted to sacrifice sentient people because they can't grasp how noble these people were in comparison to us. They viewed themselves as the stewards of the star. Their disagreement was one over duty.
If my mom ended up in purgatory to save me and I can sacrifice some cows to get her back, call me Burger King. We invent a sentient race because there would be no disagreement among humans or the sundered races if we could sacrifice animals to get our loved ones back. It would be more like if someone kidnapped some of the Viera from Fanow and wanted the secrets of Ronka in exchange for their release. Some would be like, yeah, let's give it to them and get our people back while others would be like, no, it's our sacred duty to protect that information, and the people they have hostage have made that same vow. That's why Emet-Selch is abhorred at the suggestion he'd try to free the people in Zodiark because based on his value system they are doing their duty.
Both sides are perfectly reasonable stances to hold and the solution to resolving them isn't murder everyone. She allowed Zodiark to happen because she needed him as a shield for her new world and then used his existence as an excuse to get 12 people to give up their lives and more of their souls than they seem to know they will be losing so she can kill everyone they know and love.
But let's say I'm wrong and the lesser life isn't animals. Let's say that it's sentient beings, us even, let's say they wanted to sacrifice cat boys and bunny girls. Well, whatever life they were fighting over was sundered, so RIP. And the people who wanted to protect that life and agreed with her? Also sundered. So attempting to humanize the third sacrifice to dehumanize the Ancients doesn't work because whatever the third sacrifice was she destroyed it in the Sundering. You can't save anyone by ripping them to shreds and mutilating them to the point where they are no longer recognizable and reducing their lifespan by a factor of hundreds. That's not how saving works.
Her goal was not to save the third sacrifice, her goal was to make sure the future was shaped by her hands and hers alone.