Thinking about the throne room scene in ROTJ, doesn't Anakin/Vader have to do something or have something happen to him that makes Sidious/Emperor trust him? Either the Emperor has a lot of faith in Vader or he's incredibly stupid during ROTJ when he's cooking Luke right in front of Vader. The Emperor is making a heck of an assumption that Vader would just stand by and watch as his son gets fried. I think Anakin's dark side turn has to be of his own free will. Sure, it can be due to a twisted sense of right and wrong - Lucas is obviously modelling the PT with Anakin as the typical tragic hero. There was a good thread that specifically talked about Anakin as the tragic hero, but I think Anakin/Vader has to do something to warrant the kind of trust (or at least not mistrust) we see in ROTJ.