Originally Posted by Zerimar Nyliram
The ending where the Dark Apprentice kills Starkiller isn't canon, but the Dark Apprentice's existence is canon. As to why he chose not to strike Starkiller down in the light side ending (since he was obviously so close, and was not detected by General Kota) is anyone's guess.
No it's not. Vader clearly tells him to not step in until needed. In the DS ending Vader's about to get killed, so the clone reveals himself. In the LS ending Vader is not only not in danger but is in fact even on his way to the secret rebel base.