He is not, This is a clone that was actually from the first video game of the force unleashed where Darth Vader killed starkiller to show his loyalty to the emperor. His memories are side effects to the cloning process
The answer in fact varies. There are two possible endings in the Force Unleashed. In the first alternate ending he is killed by Darth Sidious while in the second he turns into a Sith lord. By means of the Expanded Universe he is considered dead.
LEGO Star Wars: A32CAM LEGO Star Wars II: HHY382 LEGO Star Wars: The Complete Saga (XBox 360): EMP666 LEGO Star Wars III: QXY5XN The Force Unleashed: PALPATINE The Force Unleashed (XBox 360): MASTERMIND