[nx1701g]

^ It's explained in the game/novel

Vader ultimately plans to attack Palpatine alongside Starkiller and take his place as the master with Starkiller as his apprentice. No one can know that Starkiller exists especially the Imperials as they can make reports to the Emperor telling him of Vader's plan.

The Emperor developed the plan in the beginning to use Starkiller. Starkiller believes that he is meant to be Vader's Apprentice, but Vader and the Emperor came up with the plan as a means to draw out the last remaining Jedi and the leaders of the Rebellion to kill them for their attacks against the Empire. Starkiller learns of this and turns on Vader and is killed in the process, but the Alliance - because of his sacrifice - swear to always honor him and the Rebellion Logo is actually his family crest).