I'm glad someone brought this up. I was thinking that Caius was never inherently evil, just misguided. He still had a genuine cause that he followed, and took Etro's orders to protect Yeul to the max. With Bhuni coming into the story, Caius would probably turn into something of an anti-hero. Bhunivelze wouldn't exactly want the destruction of 'his' world. Instead, he'd most likely want to eliminate Etro and Muin, meaning destroying Yeul, Lightning, Serah, Snow, and Noel. Thus good Caius.
Oh he's definitely not evil. I would be surprised if anyone actually thought that o_o IMO, Caius is the most human antagonists from FF's history.
If Caius were to become an anti-hero, I think he'd probably just got a guest role as playable if at all. His goal is to protect Yeul, depending on how distorted his mindset is (he's been mulling this for hundreds of years, after all), he might consider Bhunivelze's revival and destruction of Etro and Muin a good thing, even if it meant destroying Yeul's soul eternally?
Another thing I have to wonder is how important the lives of Lightning, Serah, Noel, Snow, Hope and Sazh (and Dajh) is to this whole thing.
SPOILER FOR PARADOX ENDING (as if I haven't spoiled enough people): But Snow pretty much calls Hope one of the most important beings for humanity's future. WTF DID SNOW SEE, is something I want to know.
What about everyone else? Has Noel completed his leg of the journey with the change of this world by killing Caius and thus killing Etro? I seriously doubt it. Even if he had, I don't think he'd willing stand by.What of Serah? And Lightning? And again, Sazh. I cannot wait for his DLC, btw. I want some answers.