Twist the perceptions of "what is right" and "reality."
TALES OF SYMPHONIA SPOILERSSS OMG
Mithos and his friends and sister (Martel, Kratos and Yuan) spent a great deal of years trying to save the planet and it's energy crisis. That is, the humans were drawing so much energy from the planet, that it was syphoning all of the energy away and killing it- think FF7's original plot with the tree hugging terrorists of Avalanche, same idea. But in Mithos's quest to save the world, he decided to use a special sword to cleave the world into two parallel worlds, and make a system where one will syphon energy from the other. Neither world half would have enough energy to truly prosper, but both would trade the energy on and off. In the meantime, Martel had almost died due to their efforts, and Mithos was working on a way to save her life by using the same idea of energy transferance. What ends up happening by the time Symphonia itself takes place is that one halves of the world had learned about this sharing of energy and was being greedy to keep it maintained forever in their balance, by murdering the mechanism that would have forced the energy switch to happen. When the present day takes place, the heroes Lloyd and co. are forced with a horrible reality -> to join the world together they must destroy the mechanism that is funneling the sharing of the world's energies, which would in turn, kill Martel since her body had joined with the very thing that would join the two worlds together. They can kill the seed and let Martel live with all of the energy it's gained over the years, or they can kill Martel herself and ruin all of Mithos- the hero of olde- plans. Turns out that Martel is barely alive as is and Mithos is running out of time anyway, and the seed itself is basically dead. The only way to save the seed is to take all of the energy that would revive Martel, and any energy within Martel at the time, join them together and use that last spurt of the planet's energy to revive her.
Mithos himself has become twisted through the years. He tried to save the world, and had thought he had worked out a perfect solution. The trade off all those years ago, was Martel's death, and Mithos - a child - could not accept that. Even as he grew up, very slowly, through the ages of having lived through persecution and abuse and had spent all that time to save the planet, to see his sister starting to die. He just couldn't take it anymore. He is both a victim of our present heroes Lloyd and co, and a villain. He would rather see the planet die to revive his sister because what is the point in reviving this world that had forsaken them? What is the point in reviving a world that had persecuted them? That hadn't cared what they had done, what they had sacrificed.
Abyss and Vesperia are lorded as being great storytelling masterpieces of Tales of series in that they deal with present day politics, who has the ability to rule a kingdom, what is right and is death an appropriate punishment, and should you mess with someone's genes to make a clone. Morally are these actions correct? Symphonia (and to a degree Innocence) deal with the question of is it right to sacrifice the one for the many? Especially with what is revealed in the sequels to Symphonia (Symphonia 2 and the original tales of phantasia), can you really say Lloyd's group made the correct decision: they chose the entire planet over the life of one. The planet does not change. The two groups of people having been kept apart for thousands of years are suddenly forced together, and both still persecute elves. Those issues do not vanish overnight as Symphonia 2 illustrates. Those issues do not disappear even in another thousand more years as Phantasia illustrates. Even into the future of Phantasia (time travel is kind of a thing there), it is revealed that no matter what, the core idea Mithos had was correct: his people would always suffer persecution and live in fear and in tiny abused populations. Some of them may be able to have better and peaceful lives, but as a whole, the group wants to live isolated from the rest.
Mithos wants not only to revive his sister, a very VERY strong driving force for him, but he wants to wipe out persecution. All of the elves would rise with him to live in peace away from the humans, devoid of emotion, or feelings, and unable to be hurt, or to hurt others. Think of it as a psychological twist of Syndrome's idea from The Incredibles: when everyone is Super, no one will be special. You and I can see that Mithos's plan is doomed - the planet will die, his sister will likely die without intervention (sacrifice the whole planet's last weak energies to her), and the people (human or elves) underneath Mithos's reign will forever be lifeless (no emotions with no ability to escape that state). But we can also see that Lloyd's path was not the clear cut solution either - it resulted in Mithos's fear, his sister actually dying (arguments of her soul becoming the being that protects the seed, and subsequently the tree exist), and the persecution of his people remaining unchanged, and the planet working toward greedy ends (using the planet's power for war). Literally the actions of Lloyd's heroic group undoes all of the efforts Mithos did to stop the wars of old.
In short, analyze the past, the future, and the present reactions of isolation of a group of people. Will the outside world's treatment of these isolated people change by the heroes actions? Will it be for the better, or worse? Or will it not change at all? When one goes to extremes to stop people's ability to destroy the planet with the planet's own power in their lust for power and war, are the extremes justified?