Just as I feel having Han and Leia being separated would hurt Han's arc, I think having Luke fall to the Darkside would hurt Luke's arc. Luke was an impulsive, reckless kid who sought out adventure and excitement in the beginning. He had the makings of a hero but was pretty cocky about his achievements. He ignored the advice of his mentors and rushed to meet Vader too soon. He lost his hand in the process and had the chance to save himself by joining Vader. But instead he threw himself into a big pit likely uncertain of his fate. Later he willingly turned himself over to Vader and the Emperor in an effort to save his friends. Although he was momentarily overcome with anger and rage, he saw the error of his ways and threw his weapon aside. Rather than trying to kill Vader and the Emperor, he boldly stood his ground and faced death through electrocution. If he had misjudged Anakin's propensity to turn back to the Lightside, that would have been the end of Luke Skywalker. Forgive the TDK reference but he was willing to "die a hero rather than live long enough to see himself become the villain." But Vader did save him. Luke had truly become a Jedi Knight at that moment. He had grown from the whiney teenage kid into a responsible and wise warrior. I fully expect Luke to show even more wisdom as the mentor character in this trilogy. If he is killed off, he will die a hero. His death would serve a catalyst for the audience to hate the villain, for the new protagonists to be lured to the Darkside, and I think it would be a nice parallel to TPM and ANH with the death of the mentor.
