Right before you fight Vader for the last time, you're actually about to choose whether to fight Vader for the sake of "Personal Revenge" or the Emperor for the "Greater Good". It all hinges on which direction you run. I don't remember specifically, but I think one involves running to the right and the other running into the background or something like that. That's how you choose which ending you get. One unlocks the Light Side Jedi Robes and the other turns you into Cyrax from Mortal Kombat.