Let's just put it like this. Anakin fell, got mauled, out into a suit. That, in a nutshell, is Vader. Now on the subject of him being a sith, well he was. He was knighted as a sith lord by a sith lord, given the signature dark side powers, red lightsaber, etc. His title was never in question. The reason why he never killed Sidious? Okay, this might cause some of you to disagree, but I have a nice answer for that. The life-support system integrated in his suit is completely electrical. That being said, he wasn't stupid enough to let Sidious just short out his system by merely casting force lightning. So no, he can't kill Sidious that simply. As far as Vader turning good at the end of ROTJ, remember one crucial factor. He was hit by force lighning when he was young by Dooku. He knows first hand what it feels like and he didn't want his only osn to go through such a painfull/torturous experience. Combine that, with his love for his son, his hate for his master, and you've got a father pushed over the edge that will stop at nothing to save his son, even if he dies doing it. Because of Vader's sacrifice, the jedi could live on. Which is the irony, as he helped to kill so many of them before that point. Had Vader not stopped the emperor, the jedi would've been gone, the alliance destroyed, and the sith with absolute control of the galaxy.

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I will always be the dark side. There will always be an empire, so long as the sith never die...