You're both on the right track but a little off. Sephiroth spent all that night in Niblihem (Cloud's flashback) looking up information on Jenova. Seeing this information caused him to go insane. He tore the 'Jenova' door open in the reactor, trying to 'set' her free. Fortunately Cloud & Zack intervened, preventing him from this. More later kay?
Thanks- BudSoda02 BudSoda02@yahoo.com