Venat's motives have always been a bit shrouded. We know they had a falling out with the Occurians and helped Vayne and Cid discover how to create Nethicite in order to be on equal standing with the Occurians, but exactly why Venat chose to do this is never really elaborated on. The best we can guess is that Venat either felt sympathy for the races of Ivalice and felt they were capable of making their own way through life, or they simply saw the system the Occurians had established as unfair. A lot of this could have been cleared up if the game had developed the Occurians more, because we really don't have much of an idea on what their actual endgame was, nor how destroying the Sun-Cryst somehow stops them from meddling any further.

I agree that I felt Vayne was always meant to have a different end to his story than what we got. He's a far more conflicted figure with a pretty sad past, but then he goes super megalomaniac once you get to the Sky Fortress Bahamut, which feels really out of character for him.