Vader [Major Spoilers]
"I lied, as I have from the very beginning."
Vader claims he never intended to use Galen to destroy the Emperor; it was always an elaborate plan to draw out the Empire's enemies and have them destroyed. Plain and simple, when Vader said he lied from the very beginning, I think he was lying.
First of all, why exclude the Emperor from the plan, if the goal was to destroy the dissidents? Ah, but a line towards the end of the game suggests that the Emperor did know about the whole thing. Which was it? If the Emperor didn't know, why the hell not? If he did, why the elaborate hoax of being angry with Vader and "killing" the apprentice?
Neither Vader nor Palpatine is stupid. Why would they go through all this trouble if Vader could have simply told the apprentice early on the distract the Emperor and create the Rebellion? Or why not tell him the plan was to destroy the rebels, then kill Palpatine? He could easily kill him the way he pretended to the first time.
Here is what I think. Vader intended to use Galen to overthrow the Emperor. When the Emperor discovered him, Vader "killed" him and gave up the idea. Then one of two things happened: Vader decided to use Galen to help the Emperor and gain back his trust, or together Vader and Palpatine decided to do it. It makes much more sense that Vader was forced to change his mind. Why lie to the apprentice and say he had never planned to use him to destroy the Emperor? To crush his spirits, perhaps. Perhaps to cover himself by making it look like he has always been loyal the Emperor (you wouldn't want any storm troopers to overhear Vader saying that he had intended to betray palpatine).
Plus, it is very much in Vader's character to want to destroy the Emperor. He wanted to kill him when he found out he was a Sith. He told Padme he could kill him and take over. He wanted to kill him in the end of Revenge of the Sith, but couldn't. And he wanted Luke to help him destroy him.
"Everything I tell you is a lie. Every question I ask you is a trick. You will find no truth in me."
―Vergere to Jacen Solo