Thanks to ginja_ninja for the inspiration!
Of course, this theory about Venat is true:
Venat seeks to free mankind from Gerun and his ilk by giving nethicite to the Archadians so they may overthrow the descendants of Raithwall, the Dynast King the Occuria would put in power. Though the Archadians are a means to an end, Venat does not control them as the Occuria usually control others; Venat considers Cid and Vayne Solidor friends and is considered the same in return. Despite its differing methods and motives, it can be argued that Venat is still doing the same thing the other Occuria have always done: bestowing the power of nethicite on a mortal of its choosing and using that mortal to rule the world.
Venat did everything it did in FFXII with the express intent of getting mortals to break the seals on the espers and unleash the Lucavi on Ivalice.
The game’s lore explains at several different points that the espers are scions of darkness imprisoned by the Occuria after they tried to rise up in rebellion against them at the dawn of time. The reason Venat is at odds with the other Occuria and branded as a heretic has nothing to do with supporting humanity; it has to do with siding with the espers.
The motivation feeds Cid and Vayne about “the reins of history back in the hands of man” is a lie. It manipulates them in the exact same way the Occurian Council manipulated Raithwall and its other chosen figureheads. Vayne is just another Dynast-King, granted power and nethicite through Occurian hands.
What Venat really needed was a war. The motivation for mortals to seek great power for the sake of conquest, vengeance, or peace. As we go through the game Cid and Ashe unlock each of the Thirteen aspects of the Zodiac to gain their power.
Venat doesn’t seem to care when it’s defeated because it wasn’t; it accomplished its goal. The seals on the espers are broken and the Sun Cryst is destroyed, severing the other Occuria’s tie to the mortal realm and ability to choose a leader.
The espers for some reason are apparently unable to be resealed, and after Ashe’s lifetime now are free to act upon the world. A great cataclysm happens at some point that wipes out all non-hume races. This was almost certainly caused by the Lucavi under the leadership of Ultima. In the resulting chaos, Ultima finds her own figurehead in the form of Ajora and founds the Glabados Theocracy, setting up the plot of Final Fantasy Tactics.
The question then becomes why? What was Venat’s motivation? That’s unknown:
For unknown reasons, Venat objects to the Occurian method of controlling history by giving power to Man as a means to manipulate them. It questions the right of Gerun, leader of the Occuria, to dictate the destiny of Ivalice when by their nature the Occuria are exempt from time and do not live among the mortals.
Venat’s goals could be an allusion to Prometheus, “who defies the gods by stealing fire and giving it to humanity, an act that enabled progress and civilization”. Only here, Venat decided to give life to mankind’s path. And in an interesting twist, breaking the Sun-Cyst (a symbol of fire) frees mankind. Like Prometheus, Venat is mankind’s champion. Yet this makes him a questionable hero, along with Vayne.