"And when he finds out Luke is his son, his first impulse is to figure out a way of getting him to join him to kill the Emperor. That's what Siths do! He tries it with anybody he thinks might be more powerful, which is what the Emperor was looking for in the first place: somebody who would be more powerful than he was and could help him rule the universe. But Obi-Wan screwed that up by cutting off his arms and legs and burning him up. From then on, he wasn't as strong as the Emperor -- he was like Darth Maul or Count Dooku. He wasn't what he was supposed to become. But the son could become that."