FF7 Why did Sephiroth becomes evil?

He believed he was a Cetra and that Man had betrayed them two millennia ago, but how come he never discovered he wasn't a Cetra and that Jenova was a hostile alien lifeform while traveling through the Lifestream and acquiring its knowledge?

He got existential crisis that makes his sanity snap. Finding out where he came from makes his brain switch to overload.

His life is a lie and the truth is too much for him to handle. He reminds me a lot of Homelander from "The Boys"

Why did he turn evil? He is ultimately so mad, selfish, and delusional that he believes he is the pinnacle of existence and shouldn't have to coexist with any lower forms of life. He will therefore destroy all of those inferior beings using orbital bombardment in order to absorb the Lifestream and effectively become a god.

Thirst for power and identity crisis. He just find out he is made in the laboratory.