While it's true that he wasn't really one (too attached to the people he cared for and not enough attachement to the power that makes a good Sith, ala Bane [WHO IS AWESOME I MIGHT ADD!]), he was a very powerful force user, which made him an ideal tool. The big flaw was really trying to turn someone who Vader cared for to the Dark Side. If Palpatine, instead, had just killed Vader at that point and used the anger Luke had to turn him to the Dark Side, he would have made the perfect Apprentice. Luke would want to avenge his father. The only way to do so would be to get power... power over the Dark Side... and use that power to kill Sidious. :twisted: