We have been shown that Anakin over and over again wanted to take out the Emperor, if so then it begs the question, why didn´t Vader used some chances he had to accomplish this? 1) The exploding throne Eearly in the Galactic Empire the Heinsnake Cult planted a bomb in Palpatine's throne, he had not detected the bomb, he was going to end up killed by the bomb if Vader did not intervene. Even if he survived it would had been the perfect chance to take on the Emperor, a Palpatine severily damaged by the bomb would stand no chance to Vader. The issue is, Vader actually SAVED Palpatine, why? I mean he could had done away with him by doing nothing at all and become Emperor himself. 2) Starkiller Pretty much we were told Vader only took Galen Marek on orders from Palpatine and he never planned to defeat the Emperor with him, why? It was a good plan actually, even if the Emperor planned it for something else Vader could had twisted it to his advantadge, but no, he actually was a loyal dog the whole ordeal. Granted perhaps when the Emperor came to them the first time Galen was still too weak to take on him. But at the end Galen was strong enough, when he came to the Death Star when Galen said "You will never be free without me" Vader just agreed and teamed up to fight the Emperor? There is no way he would had been able to fight Vader and Marek at the same time. Well so that's it for now, Vader actually had 2 good chances to defeat the Emperor, one of them with just required him to do nothing, and he didn't took them. Why? I never have found any explanation for this.