Let's talk about Caius and Yeul, shall we?
First, who in the world thinks it's a bad thing for Caius to be obsessed with protecting Yeul? Seriously. Is it the purple hair? Is it the fact that you fight Caius over a dozen times (counting Paradox Endings and DLCs)? What in the world is wrong with Caius loving Yeul to the point that he wants to destroy time itself?
Not to get too existential (but to get slightly existential), Caius is basically being told that his daughter is destined to die before she can reach 20 years old. He starts out as a Guardian, impressing the Goddess so much that she gifts him her immortal heart so that he can protect Yeul for eternity. His job has now become to eternally protect and care for Yeul, but he's also being told that Yeul will never be allowed to have a long and fulfilling free life. His job is to protect her, but the one thing he can't protect her from is her fate. If she sees a vision where she has to die, Caius is not allowed to stop it. He raised and protected this girl just so that she can die to keep the timeline stable or whatever. His job is to make sure that she dies the "correct" way for the sake of time. Can you blame him for being driven mad after being forced to escort Yeul to her destined death generation after generation?
Caius loves Yeul. He is not in love with Yeul (you creeps of the internet), but he loves her. He loves every version of Yeul that comes along like his daughter, no matter how different she is, no matter how she is destined to die. He remembers every one of them, he loves every one of them even with how many he meets and raises and loses each generation.
"Although they had the same soul, every one of them was unique! A Yeul who dreamed of travel! A Yeul who loved to sing! A Yeul who collected flowers. They all died. All of them, before my eyes!"
Serah collapses after he says this, even though she's just standing to the side. Why? Because she's horrified. In 400AF, a single Yeul had to die because of Serah and Noel's presence - for the first time, Serah realized that changing time is hurting someone. Serah is responsible for that Yeul dying. Every change in timeline, even if it's to fix it, is forcing an innocent girl to walk into the arms of the nearest monster so she can die and keep things from getting worse.
Serah was horrified that Yeul just walked to her death willingly, like Yeul sacrificed herself just to allow Serah and Noel to continue their journey, but Caius has seen Yeul accept her fate hundreds of times. He paid attention. He remembers every Yeul, their dreams, their preferences, their favorite activities and favorite experiences, and he's had to watch them die just like Serah watched 400AF Yeul die without being able to do anything.
Caius got so tired of being immortal, but the main reason was that his entire existence was to watch a girl grow up and then die - often in his arms because he has to protect her up until that point. It finally ended at 700AF, Yeul's cycle of reincarnation was likely at its end since there were no more humans, and the next Guardian was born who might finally be able to end Caius as well. He hoped the cycle would finally end, but Noel was too weak to kill him. Yeul believed in the future, and that was awful to Caius because it meant that this wasn't the end. Yeul would keep living and dying for others, never living for herself, and Caius would be forced to keep watching it and letting it happen forever.
"Her only purpose is to die over and over! Even though she can see the future, she's not allowed to escape her fate! She is born knowing that she will die before she has truly lived! Countless deaths, without a life to give them meaning."
Let's talk about Noel too, because his relationship amounts to only truly knowing one Yeul but overall valuing her just as much as Caius.
Now, remember that Noel is having memory problems for the entire game. His memories are in flux, sometimes leaving him, perhaps even changing without him knowing it. That's why, when he's killed in the Void Beyond and sent into an endless dream, his dreamworld sends him home. The end of the world is depressing, it's dark and sad and he's only holding into hope for Yeul's sake - to see her smile, to find more people so she won't have to be alone. Noel's dream is to see his Yeul again. Even if he has to go through losing his life again, this is the world that he knows and the world that he was slowly losing as a result of his memories vanishing.
Right after you wake him from his dream world and enter the lighted (real) 700AF (since you can eventually reach it from the Historia Crux, I believe the lightened version is the actual 700AF time period; they escape the Void Beyond by translating Noel's dream into a real time location at the end of the world by slaying the Gogmagog), Noel says that he "can't remember what happened to her", despite just remembering Yeul's fate dying young to her vision - this is the place we learn that visions eat away at a seeress' life and Serah is one too.
This is probably lost in the English translation, but I interpret this as Noel knows in concept that Yeul died because of her visions, but he can't remember details of the event itself. Right after Yeul dies in the dream, Noel pauses to give the narration of how he learned why Yeul died in his arms. How did he learn that Yeul's death was a result of her visions? As far as Noel was concerned, in the cutscene we viewed, Noel walked up to Yeul putting a prophecy in an Oracle Drive, she collapsed in his arms and told him they'd meet again, and then she died. Caius was already gone; no one could have told Noel that Yeul didn't just randomly keel over. Maybe he could have guessed that her vision was related to her death, but he could have very well just assumed she had a particularly terrible vision that killed her, not that each one takes a part of her life.
When confronting Caius in the final battle(s), Caius assumes that Serah doesn't know about visions killing her - he is proven wrong. When Caius declares that Yeul is bound by a curse, Noel asks "What curse?" I interpret this as Noel not thinking that Yeul's visions and rebirth are a curse. It's tragic and scary, certainly, but Serah's situation is what Noel is more worried about. Unlike Yeul, Serah won't be reborn; she gets one life's worth of visions and that's it. Yeul has to die over and over, but Noel only focuses on the fact that she gets to live over and over. Her lives are short, but even when she was at the end of the world with only two people left to accompany her, she kept smiling. Noel concludes that she must have made the most of every life, because even though her lives were short, she lived so many that she had more time than anyone could have lived in a single long lifespan. Noel does not see Yeul's endless death and rebirth as a curse.
When Serah is brought to her knees at Caius's words about Yeul's deaths, she is able to rise again because Noel declares that Yeul's deaths weren't a curse. "She knew we'd meet again. Think, Caius. Think. Was it really a curse? Was it forced on her by Etro? Do you really think that Yeul wanted to die, and not come back? Of course not. Yeul wanted to come back. Every time she died in your arms, she wanted to come back. She knew her next life would be short. She knew! Because she wanted to see you! Again and again, without end!"
The Eyes of Etro are a curse that take away a seeress' life with every vision, there's no denying that. When the timeline changes or is put at risk, someone has to bear the weight of the consequences. If Yeul hadn't been reborn, it's likely that a random candidate would have been chosen each time a seeress died, similar to how Serah gained her Eyes when her personal timeline was altered and her subsequent journey was the catalyst to the entire timeline being put in flux.
But Yeul choosing to live again and again must have been her own choice. She wanted to keep others from suffering, but she could also live what amounted to a long life, living in every era, seeing Caius again, having new experiences and new interests and new adventures as the world changed with each life she lived. She didn't need immortality; she wanted reality, no matter how cold and harsh it could be. The sad moments were worth the happy visions and happy futures.
Caius was the one consistent thing about each of Yeul's lives, the one thing that didn't change. She lived life after life in every time zone, probably missing large chunks of time and being disconnected from her reality because a seeress needs to be careful who she interacts with (see the fall of Paddra for an example of how her presence is dangerous). Of course she grew attached to Caius as an anchor, the one consistent thing about all the times she's lived.
Notice how there are scenes depicting both Noel and Caius alike crying over a Yeul who is smiling when she dies. Noel was the first Guardian who gave Caius hope that Noel could kill him; Noel was the first person that a Yeul valued besides Caius because she had a vision that she was going to see him again. Yeul likely lost everyone else in each of her lives; by the time she's reborn, all the people she might have become friends with are gone and she's isolated from everyone except for the Paddraen tribe and the Guardians. Caius and his immortality is the only one that she always knew she'd see again - until Noel happened. Yeul died in Noel's arms just like she had so many times in Caius's arms, but Noel chose to value the idea that he'd see her again, not dread it.
As revealed in Lightning Returns, the Unseen Chaos (a different type of Chaos from the regular one that makes up all souls; a more volatile version that destroys) is made up of Yeul's souls specifically. The Unseen Chaos protects, empowers, and even revives Caius because it is Yeul looking after her Guardian just as Caius was looking after her. Yeul is the Chaos that floods Caius after Noel's speech where he has to strike back as Caius doesn't believe his words, the Chaos that transforms Caius into the Jet Bahamut for the final boss of XIII-2. She was desperate to keep the one consistent thing about all of her lives with her. No matter what Caius did, how "I don't want to please her, I want to save her", and how he destroyed the world even when she just wanted to see visions of people living happily, she can't let him go.
Ever had to make a decision and kept going around in circles about the pros and cons, what you gain but what you have to sacrifice for one decision then the next? Imagine every thought you had existing as a separate entity - still fundamentally you, because you came to that conclusion at one point or another, but conflicting and arguing over which opinion mattered most. That's what Yeul is, hundreds of people all with different thoughts and opinions but who fundamentally start out as the same base person.
She hates Caius for some things, but that doesn't mean she doesn't also care about him and need him. For all he does, Caius cares about Yeul, telling Lightning that Yeul cannot go to the New World without destroying it with her Unseen Chaos, so he's fine accompanying her to make sure she isn't doomed to be alone forever.
Caius is very evil at times, driven to his goal with absolute focus. But his greatest flaw is being completely dedicated to Yeul. In Oerba 200AF, that Caius is not hell-bent on destroying the timeline yet. That Caius is the first one, living through all of the lives of Yeul, before 700AF happens and he declares he's going to slay the goddess. That Caius is dedicated to eliminating time-travelers because "To change history is a sin." He bows to Yeul and accepts her judgment when she declares that they can remold history as they desire. The only reason they go to Augusta Tower is likely because it is the sight of a massive Paradox; that 200 AF Caius is dedicated to stopping all Paradoxes to hopefully extend Yeul's time.
However, Yeul gives Serah and Noel the artifact that allows them to resolve the Paradox, killing Yeul with the vision in the process. Notice that Caius doesn't see Yeul happy at her vision where everyone's smiling. He just sees her dead. This is how Caius came to want the timeline destroyed; he tries to keep history in line to keep Yeul from having visions, but she's completely fine with dying if it means a happier future. Nothing he does can save her. Nothing except destroying time and fate itself.
In the end, he died in Valhalla to be with Yeul forever, he stopped her cycle of reincarnation, but it didn't save Yeul because her Chaos was too strong. She destroyed the world, not even on purpose because she can't control it. It's just her existences themselves that cause destruction. Time was destroyed, Yeul no longer was born into new lives and no longer had to suffer visions, but in return, the two of them live in an eternal half-dead, half-living state within Etro’s Temple. Yeul has Caius with her forever, Yeul no longer has to die, and yet it’s all still wrong.
So really, the main enemy of XIII-2 was Etro, who loved humanity so much that she broke the timeline, gave a man an immortal heart to the point that it drove him insane, allowed a girl to be reborn so many times that her souls became a living weapon, and who gave Serah the Eyes of Etro even though Noel can travel the timeline without needing them.
Final theory of the post: what if 700AF Caius deciding to use his power to change the timeline is what began Yeul's many deaths and rebirths in the first place? If he hadn't begun messing with the timeline to try and destroy it, what if Yeul rarely would have had visions without all those disturbances? She wouldn't have had a short life, she wouldn't have wanted to be reborn. Random people would have still been blessed/cursed with visions every now and then, but not to the point of always dying young. What if Caius created his own suffering with his vendetta? Caius is the eternal paradox, the one that started it all and who maintained the distortions so long as he lived.
