Well Sephiroth isnt evil.
He is and stayed a soldier !
He followed the path he beleived in.
He was a Cetra.
His reason ?

Cloud: Normal members of SOLDIER? You mean you're different? H... hey,
Sephiroth!

Sephiroth: N...no...... ...Was I?

Sephiroth went sword crazy (what I'm trying to say is that he took his
Masamune and kept slashing things).

He his disturbed already by the fact he could have been a lied too .

He go find out about himself. Not because of Jenova but because of what Cloud brought up !
Refer to Sephiroth Studies

He called Cloud a traitor and here is why !

They took that which the
Cetra and the planet had made without giving back one whit in return!
Those are your ancestors.
Refer to He explain some to Cloud


He doesnt see the others now as ppl but nothing more then Creature.
Now he want to avenge his ppl the Cetra !
His life is the chance to set thing the way they were suposed to be !

Refer to When he find Jenova



Note: When i say "Refer to ???" it s from my last post. And the ??? will be in Bold + Italic in my last post too making it easy to spot.