Vader, realizing Sidious had been manipulating him all along after his admission that he could not save Amidala, intended to help destroy the Jedi because he knew they would never understand Windu's death in the context it would help save the one he loved against the will of the Order's tenets. He planned to assist Sidious in his plot to bring the galaxy under his supreme rule only to turn on his new Master once the Jedi were extinguished. Vader knew that the Order would object to the rule of a murderer once he took command, despite his intention of bringing stability to the galaxy's citizens through that means. He resolved that the actions he would take that night were necessary for his goal to come to fruition.http://starwars.wikia.com/wiki/Operation:_Knightfall