After thinking about this some more, I've also come up with some arguments for Sephiroth.
Historically, the story of Sephiroth is one of those things where it was predetermined that this guy would become one bad a## dude. YEARS OLD SPOILERS!!
Sephiroth's mother is injected with cells from an alien being known as Jenova. She never even held her baby, he was taken away after he was born. Shinra raises Sephiroth to be a super soldier, but he knows he's different. There is no information about who he is that's given to him. He's lied to all his life. He's only told his mother is Jenova (not his real mother) and then the only person he really respects he comes to learn is kind of a whack job.
Sephiroth eventually has an encounter with the leader of AVALANCHE, Elfe', and then this entire process of experiencing emotion and who am I starts to swallow him. He's lost the only friends he knows at this point. Both were killed (seemingly) and in his travels he discovers these pods with one labeled Jenova.
He loses it.
What do you do? You're the strongest person on the planet. You've been lied to, used, your friends killed, your relationships severed, you're lonely, sad, abused, not knowing. What do you do?
Sephiroth goes to a mansion where he studies the notes of the Jenova project. More betrayal. No one ever told him the truth. Now he believes he's a true Cetra (what Jenova is) and with everything that has happened, the humans betraying the Cetra 2000 years prior and now this what do you do x 2?
You make them pay. All of them.
He falls into madness.
The events of the games take place and there's never a recovery to Sephiroth, but because of how the story is released you never realize how human Sephiroth truly was. That's why monsters and legends aren't as scary as the actions of man.
