in this game you really need to realize everyone has there own personality and thoughts even the Occurians. one might not agree with the rest so it goes and does what it wants.
Venat wanted to take the power away from the Occurians. maybe it had its own intentions besides just give power to life on invalice. maybe it wanted to get rid of the occurians so it could have control over the planet and to do so it manipulates the minds of 2 major parts of the ruleing empire. Vayne and cid.
as for vaynes beginning speach its nice to see that even villains have there nice side but it was more then liekly jsut a trick to gain the trust of the people (polotics). Vayne kills his 2 brothers and his father and the senate too i think (corruption in polotics) as for the judges they dont all agree with Vayne as seen. everyone has there own personality and views. also we cant really think of vayne and venat as good guys because they decided to attack a helpless rabanaste.