Everyone just assumes that if the Emperor hadn't “interrupted” Luke when he was fighting Vader in RotJ, that Luke would have kept going and turned to the dark side. In actuality, we don't know whether he would have or not. Personally, I think it would be pretty lame if that was the only thing stopping him. Plus, it makes the Emperor look really dumb. But then, with pretty much any perceived problem in a story, I tend prefer just about any explanation other than “Such-and-such character was just stupid.” Even if that is the easiest explanation. So why was the Emperor so darn confident in RotJ? Well, I think it's quite possible that the Emperor had foreseen Luke's turning to the dark side (and joining him?), even though he never outright says so. And if the Emperor's foresight had pretty much always been right up until now, it would make sense to be confident about it. As we all know, he turns out to be wrong (I love the uncomprehending stare he gives Luke for a few seconds after Luke defies him), but still. What do you think?