Fayth=Evil???

[Firo Volondé]

I doubt it. If that was the case Yuna would not have needed to summon her aeons and would just need to kill the previous core ((SPOILER)Jecht). If Yuna never summoned he would have nothing to hide in and would have confronted them.

All Yu Yevon's power came from Sin, a product of the Final Aeons. The battle with BFA was when in his state before Yu Yevon possessed him. Yuna called her aeons to defeat the fayths, since Yu Yevon also had access to the fayths.

If he was weak, that questions the logic of the final battles. If he was strong originally the logic works and the aeons were needed to weaken him sufficiently.

You have infinite Auto-Life, which not only questions the logic of the final battle, but kills it and dumps its body in the river. If he was strong, he wouldn't have needed to protect himself with Sin.