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.