Shepiroth wasn't always evil but started out nice until he read about his mother. I warn you the folloing is a spioler but it explains alot. If you don't want to know turn back now.
Sephiroth was born to the ancient Jenova, when his mother died he was taken over to his second mother. I forget her name and Vincent took care of him until he was passed onto Hojo and became a test tube baby. He then worked as the head soldier of soldier until he went to niblhiem. Upon discovering the hidden room in the back of the mansion he learns of the Cetra,or ancients, and plans his revenge on the people of the planet.
