That's not exactly what I would call 'defeating' him. If Vader hadn't stepped in, Luke would have been a Light Side corpse, and the Emperor would have shrugged and moved on; certainly not happy at the outcome, but really none the worse for it. The Emperor failed to turn Luke, but I'm thinking 'defeating a Sith Lord' involves something a little more important than inconveniencing him.
Luke beat Vader. Vader beat the Emperor. Luke did not really defeat the Emperor.
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well in a way he did. if you think about it, it was because of him vader turned back to the light and through the emperor down the shaft. so inderectly he did kill palpatine.
i got a westion fo you guys, who builds a throne room with a big shaft in it?