Both of them wanted to become a god and destroy all life of the planet they lived on. At the core, they wanted the same
Sephiroth and Kefka are
completely at odds in terms of their motivations.
Their paths intersect and have similarities but those similarities are superficial.
Yes, both individuals had schemes that involved killing on a massive scale. However, Kefka was murdering people because he was a nihilistic omnicidal madman, who reveled in the sheer thrill of destruction. Kefka's ascent to the title of god of magic was not intended per se, it just happened as a consequence of gaining more and more powers from Espers he drained. He didn't so much pursue being a god, so much as it was the end result of taking more and more magical power so that he could enact his sadistic whims on others. He only lived to destroy and hurt people. He says at much at the end of the FFVI. He only wishes to destroy until everything is destroyed. He sees no purpose or value in life or living.
Sephiroth strives to ascend and
become a god. He discovers this potential achievement via the knowledge he absorbs from the Lifestream and decides
that is the path he wishes to pursue. He kills for
that purpose. (He also kills because he hates humanity and the planet but in regards to his scheme in FFVII, that's his end goal). Sephiroth intends to absorb (or corrupt) all of the Lifestream, make it his own, and
then once he's done, perpetuate the cycle and find a
new planet and repeat this process.
Sephiroth is perpetuating and building on the cycle Jenova seemingly played out in its existence. Finding worlds, killing all life on them, turning it into a dead husk, riding it as a meteor, and then finding a new world to torment. However, Sephiroth has higher ambitions than predatory slaughter. He wanted to become a god, and absorb that spirit energy, because he discovered that ability.
If Sephiroth were to
just be a carbon-copy of Jenova, then yes, Sephiroth would only wish to kill, replicate, and wipe out all life. But Sephiroth's experiences and higher evolution, gives him ambitions beyond just an eldritch alien from space. His predilection for violence and killing however is
probably (and this is just a theory of mine) an inherited trait from Jenova, since Jenova is instinctively a murderous, violent creature. His hatred for humanity also probably stems not just from his experiences and discovery of his life being a lie, but also as result of his latent Jenova connection feeding on that negativity and amplifying his impulse to be a cold-blooded killer. Those stressors most likely created a toxic storm that resulted in Sephiroth leaning into his Jenova legacy, embracing it, and building on it to become something more than Jenova. Yet carrying on it's cosmically lethal legacy.
But in the end, Sephiroth's not killing for fun. He's killing for a purpose.
And enjoying it along the way.
though that's no excuse, of course, especially for what he did long after he died.
Very self-sabotaging
