No Bad Guy, but Bad Situation?

[Crop]

So...aside from Seymour being all weird and wanting to kill everyone, is there a major antagonist in the game?

As I've been playing through it, I don't think there is. They seem to say that Yu Yevon/Sin is just going on instinct now and can't act any other way, I just killed Yunalesca and she genuinely seemed to think that there is no other way to beat Sin other than the Final Summoning.
I know Yevon is corrupt but they just seem to be like a government wanting to remain in power. I'd probably say they're the most evil in the game, but come on, they're no Shinra.

Don't get me wrong I love the game, but usually with Final Fantasy you have one main villain who is the main antagonist that the world is in danger from. Correct me if I'm wrong though, I'm no expert on the game.

[Cloudane]

Quite true that, I think Seymour is the main "bad guy" and Yu Yevon / Sin is more of a Necron type character that is just "there" until you ensure he's not (but a bit less out of the blue)

[Sephiroth]

Seymour is the main antagonist and villain. He just is not the final boss for a more dramatic storytelling when it comes to something more important. Could he have appeared after the final battle, absorbed Yu Yevon and become the ultimate enemy? Sure. But the story was supposed to focus on some other aspects. They could have been the main aspects even with him being the final boss but then it would not be exactly the same dramatic thing.

I consider Sin to be the true villain.

[Sin] is just an Id. It cannot be a villain.

[Karifean]

I consider Sin to be the true villain. Over the entire game, its devastating power is showcased (Zanarkand, Kilika, Operation Mi'ihen) and the objective of the game is to rid the world of it. It all builds up to a final battle where the world may finally be freed of the destruction Sin causes.
Unlike Seymour who shows up whenever he feels like it.

[kotora]

It's a good step away from the good-evil binary of the previous games. Ambiguous antagonists are much more interested than some badly-written generic card-carrying villain who's just doing things to score evil points like we've seen in the previous games.

[espritduo]

The "Spiral of Death" is the main antagonistic force in FFX. Everything and everyone that we consider "bad" are merely victims of this process. Yu Yevon created Dream Zanarkand to preserve the memory of his city, which he knew was doomed. There's nothing evil about that. He created Sin to protect himself while he summoned this Dream Zanarkand. He instructed his daughter, Yunalesca, to keep Sin alive in order to maintain this armor, via the Final Summoning. The Yevon religion sprang up in response to the fear of the god-like Sin, with Yunalesca helping to perpetrate the religion in order to further her duty as the maintainer of the Final Summoning. The current state of Spira (which takes its name from this theme of Spiral of Death), is one in which the ignorant follow Yevon, Yevon believes what Yunalesca believes, and Yunalesca believes there is no way to save Spira from Sin, and that the best anyone can do is keep it at bay be periodically defeating it while keeping technological progress (which Sin is programming to attack) at a minimum. Yu Yevon himself gave up his entire body and soul to the summoning of Dream Zanarkand, and is now nothing but a mindless husk that exists solely for the one purpose he originally intended - keeping Dream Zanarkand alive.

No one here is evil, they simply believe there is no other way to survive except to continue the Spiral of Death. That is why Yo Mika is so appalled at Tidus's actions when he learns they've killed Yunalesca. He is so overcome with grief at what he believes is the imminent doom of the world he was trying to protect, that he sends (kills) himself in a gesture of total hopelessness. This mindset is the same for everyone in Spira.

The only two forces in Spira that are not a part of this Spiral of Death are Tidus and co. and Seymour. They both wish to go against the Spiral, but do so in very different ways. Tidus goes against it by daring to end the Dream and defeat Yu Yevon himself. He is able to see this possibility because of his unique existence outside of the Spiral altogether, being one of the blissfully ignorant denizens of Dream Zanarkand who has not spent a lifetime being indoctrinated by the idea of an unbeatable Sin. Also, tje Fayth that are being forced to summon Dream Zanarkand, being tired of the Spiral of Death they're a part of but unable to do anything about themselves, give him a healthy push in the right direction on more than one occasion.

Seymour, being a more tragic character who has suffered far more loss and betrayal than Tidus, believes there is no hope of salvation, even if Sin is defeated. He has completely lost faith in humanity, and believes the only way to relieve both his pain and the pain of all people is to create a world with no people, and thus no pain, at all. He's misguided and probably insane with grief over the loss of his mother, and more than a little self-absorbed, but only a little evil, as per the prerequisite "gotta have somebody that's evil" quota of your typical save-the-world plot. So, Seymour would be the "most" evil person in the game, but he's more a mixture of selfish and unhinged than evil, really.

[Pete for President]

Very well written Espritduo.

[Skyblade]

He instructed his daughter, Yunalesca, to keep Sin alive in order to maintain this armor, via the Final Summoning. The Yevon religion sprang up in response to the fear of the god-like Sin, with Yunalesca helping to perpetrate the religion in order to further her duty as the maintainer of the Final Summoning.

Where did you get this from? I don't remember any of this even being implied.

The Final Summoning is not needed to keep Sin alive, it's been the only thing, across thousands of years, that could remove Sin, even if only temporarily. Had there been no Final Summoning, Sin would have remained alive with no threat to it at all.

Yunalesca crafted the Final Summoning (and her husband went along with it at the cost of his life) because she believed that it would eliminate Sin, who had gone from being body armor for her father to an insane killing machine slaughtering millions.

It was only after she died that she came to believe that Sin can't ever die, and I don't really think that even she believes that Sin shouldn't be permanently destroyed, any more than Maester Mika did. They just both think it's impossible.