Uh what? Who says they need a "legitimate" reason? That's subjective and there are MANY villains in media who rage against humanity for much less of a cause and for no cause at all. Who ever said villainy and evil had to be logical and relatable? A villain can be evil just because, or for purely sadistic and selfish reasons. Sephiroth is not a victim here, nor is he meant to be identified with.
I think Seymour is a much more fair example. Sephiroth's motivation and angst however, is much more internal and psychological. While Seymour's cause for turning against Spira is more tangible in nature, Sephiroth clearly states he felt different from everyone else. A clear allusion to feeling isolated and removed from people at large.
Sephiroth: …I've always felt since I was small… That I was different from the
others. Special, in some way. But…not like this…
Then you have Crisis Core. Which reveals even more how removed and isolated Sephiroth was from his peers. Sephiroth hardly interacts with fellow SOLDIERs and his only friends are Zack, Genesis and Angeal. The game paints a picture of a very socially inept and lonely Sephiroth, with one email even saying
With the main SOLDIER 1sts gone missing, and Sephiroth
not being the type to look after subordinates,
I'm thinking it's you who's gonna lead us.
You're moving up the chain!
And I'm following you until the end, Zack!
Clearly, Sephiroth isn't seen even by his subordinates as one who would even look after his subordinates. Despite his portrayal as the poster boy for Shinra and a hero, he has no real substance in terms of identifying with his men. Then there's this.
Sephiroth is known to value his private life, but there was
a time when he had two best friends.
Sephiroth is not known to be sociable or well adjusted. It's apparent that the source of his maladjustment is not just through the realization of his origins but his lack of connection with humanity as a whole. He is alone. I don't think anyone is trying to justify Sephiroth's path at all. It's not something that needs justification. It has a reason. And that reason is his detachment with humanity at large, and the fact his life was a lie. It created a swell of rage and hatred towards everything which in turn led to him turning against everything.
And it's not a strawman, my point in saying that is you're expecting Sephiroth, a video game antagonist for one game, to have as much complexity and justifiable exposition as a comic book villain who not only has been around since 1963, but has had decades of social commentary, exposition, and narrative focus injected into his character. It's ridiculously unfair.
And yeah, FF characters are engrossing but you can't be serious in expecting them to have as much narrative focus and clarity as a character that has been written and fleshed out as much as Magneto. That's like comparing Ganondorf to Lex Luthor. Yeah, they're both villains but hardly on the same level at all.
@munatik
That's your opinion and not all villainy can be morally ambigous or justifiable. That's hardly realistic. The fact of the matter is, is that some people do bad things for purely selfish and unjustifiable reasons. Lust for power and vanity are just as real as individuals who dedicate themselves to misguided crusades and thus end up doing bad things to good people. All villains can't be "fallen heroes." Villainy comes in all shapes and sizes, from Lex Luthor to Magneto to Brainac. Saying one villain is better than the other beause of their motivation is subjective and completely ignores the other facets of evil. If you can justify a villain as Kefka, who has no motivation for his actions other than the thrill of doing it, then you should be able to step outside your box and understand the internal psychological pressures that cause Sephiroth to abandon his hero mantle and for his own selfish and sadistic reasons, try to fashion himself a god of the planet. You make no sense saying Kefka has more justification in terms of being "done right" than Sephiroth when if by your logic, they need a reason to be sympathized with. Which was never the case to begin with, with Sephiroth.