Claro que ele é
A natureza da relação entre o Mestre Sith e o Aprendiz Sith recebe tratamento mais explícito no romance canônico Lords of the Sith . De acordo com os ensinamentos de Palpatine, o Mestre só deve ser derrubado quando não for mais strong; isso é algo que, a partir dos eventos deste livro (que acontecem 14 anos antes de Uma Nova Esperança ), nunca foi um problema:
Soon after destroying the Jedi, the Emperor had told Vader that he would one day be tempted to kill him. He'd said that the relationship between Sith apprentice and Master was symbiotic but in a delicate balance. An apprentice owed his Master loyalty. A Master owed his apprentice knowledge and must show only strength. But the obligations were reciprocal and contingent. Should either fail in his obligation, it was the duty of the other to destroy him. The Force required it.
Since before the Clone Wars, Vader's Master had never shown anything but strength, and so Vader intended to show nothing but loyalty.
Lords of the Sith Chapter 2
Vale a pena ressaltar que, pelo menos uma vez, Vader não acreditava que ele iria querer matar Sidious:
Perhaps Vader would attempt to kill his Master one day. Sith apprentices ordinarily did. They must, if they were trained well. An apprentice was unquestioningly loyal until the moment he wasn't. Both Master and apprentice knew this.
"But our relationship is different, Master," Vader had said then.
Lords of the Sith Chapter 2
No entanto, eventos posteriores no romance deixam claro que ele sente a tentação:
"The weak are found out and killed by the strong."
"As it must be," Vader said.
"As it must be, indeed," his Master echoed.
"But sometimes the strong mistake their strength," Vader dared to add. "And so
demonstrate weakness."
"Do they?" his Master commented, and Vader said nothing more.
Lords of the Sith Chapter 13
A thought flashed through Vader's mind, a stray thought, just for a moment: his Master dead, Vader ruling the Empire, the galaxy, unconstrained by the leash of an old man...
[...]
"Lord Vader," his Master responded, and chuckled. "Enjoyable, no? Did you consider allowing me to die to realize your own ambitions?"
Vader didn’t even attempt to lie. "I did, but only for a moment."
Lords of the Sith Chapter 16
Então, sim, apesar de sua lealdade ao Imperador, Vader está definitivamente pensando como um Sith adequado deveria; ele é leal apenas enquanto seu Mestre for strong - pelo menos até rejeitar o Lado Negro, quando se tornar desleal novamente.