The motivations of Vayne and especially Venat weren't really explored, and the ending was horribly cliche.
I disagree here. Vayne (and Venat)'s intention was repeated several times: to free humankind from the grip of the Occurians and write their own future. In a way, Vayne can be viewed as a tragic hero, sacrificing everything for this goal. He felt that coming a noble and mighty house of Archadia forced him to do such a thing. Still, he was quite conscious what he was doing was wrong but believed it needed to be done for his kind to have the chance to be free of those Undying freaks. He also placed all his hopes on Larsa and he ensured his brother would survive no matter what.
Why Venat broke away was explained as well: He didn't like the way his brethren continued to play gods even though their time had passed.