Yeah, Kefka was messed up but its the direct actions of Magitek Infusion that causes him to become the deranged, sadistic and vulgar character he is in FFVI. He was abused, withdrawn, and sad before, but then he went utterly mad when the Magitek infusion buggered up.
And yeah, Golbez had darkness in his heart, but it was never enough to make him try to erase humanity so that lunarians could colonize it. His darkness merely opened the door for Zemus to control him. Zemus is the real brains behind Golbez's body.
And that's the dynamic thing about Sephiroth. It isn't just one thing. It's the totality of it all, and Sephiroth making the choice. Sephiroth CHOSE to embrace his darkness and turn his back on everything. It's why the walking through the flames is such a symbolic statement of Sephiroth. When he sees Zack, he just smirks coldly and as if embracing hell, he walks through it to continue the madness.
Sephiroth's attachment to Jenova and her ideology pushes him, but when he clearly quotes feeling "different" and summarily calls everyone a "traitor" and turns his sword to his friend, clearly that's not just madness. Sephiroth has drawn a line between "me" and "them" and chosen to turn against everyone. It's not just him snapping, its him alienating himself further.
It's a mixture of the two, but clearly there's more psychology and choice behind it. The pain and shock of finding out your life is a lie is the trauma that really drives Sephiroth over.