The Ancients are allowed to hold a grievance against Venat. They are not allowed to wipe out all of creation because they are desperately grasping for a past that can never be reclaimed. Frankly, describing that as "seeking justice" makes your entire stance suspect.
There are plenty of plausible reasons that Venat didn't explicitly lay out Zodiark, Meteion, and so forth. The most brute force is that she knew it wouldn't matter, we're in a closed time loop and a future that produces a WoL that will travel back to Elpis is going to happen. That is also not a terribly satisfying answer, but it's notable that if you go with it, Venat still tries to convince the Ancients to reject Zodiark. She knows it's hopeless, but she still tries because she loves her people and their world.
There are other reasonable excuses. Emet-Selch's defining character trait is that he's stubborn and reluctant to accept new ideas, especially one as wild as a sad space bird is the real threat we have to deal with. Telling the Convocation will mean telling Hermes which risks him deliberately sabotaging the effort to stop Meteion. And Venat has literally no proof of her claims which again are pretty wild. But more to the point, Meteion isn't why Venat sundered the world. She sundered it because the Ancients chose slavery over freedom.
Let's go back to the whole genocide thing. Because you're right, genocide is bad. And the writers of Final Fantasy XIV would agree with you! Ishgard trying to wipe out the dragons and vice versa is bad! The Garleans trying to wipe out everything that isn't Garlean is bad! The sin-eaters trying to wipe out Norvrandt is bad! Emet-Selch trying to wipe out, uh, everything is bad! And the Ancients deliberately genociding themselves so they could cling to their tainted utopia is bad. If you want to argue that Venat was preventing the Ancients from freeing their kin from Zodiark, you have to acknowledge that they were happy to keep throwing souls into him as long as they could get their paradise back. That is what Venat sundered them from. She didn't object to the summoning of Zodiark because, yeah, there was no other way to stop the Final Days. She objected to the Ancients enslaving themselves to him. It was absolutely a genocide of the Ancients and the writers assumed that people would realize, based on FF14's previous anti-genocide stance, that it was a horrible, monstrous choice. But that with the context of her beliefs and the events around her, there were no good choices.