Also, just to further that and nail down the thread subject:
"Anakin will be taken over by dark forces which in turn destroy the balance of the Galaxy, but the individual who kills the Emperor is Darth Vader - also Anakin. The tale meanders and both the prediction, and Qui-Gonn are correct - Anakin is the chosen one, and he did bring peace at last with his own sacrfice. Luke couldn't kill the Emperor himself, but he could make Anakin reflect on his life and kill the Emperor"