We are made to think that it is unclear where Spock's loyalties lie:
KIRK: Spock! My only concern is getting the ship back.
When that's done and Sybok is in here, you can debate
Sha Ka Ree until you're green in the face. ...Until then,
you're either with me or you're not.
SPOCK: I am here, Captain.
KIRK: That's a little vague, Spock.
This may have been related to the early idea behind the script that everyone would turn against Kirk (via Memory Alpha):
William Shatner's first outline for this film was entitled "An Act of Love" and, according to William Shatner's Star Trek Movie Memories, would have been a much darker tale and would have seen the first true falling out between Kirk and Spock and McCoy. Also, Spock and McCoy would also have joined with Sybok, leaving Kirk alone. This was changed when Nimoy absolutely refused to play that, stating that there was no chance whatsoever that Spock would ever turn on Kirk, especially after what Kirk risked and sacrificed for Spock in Star Trek III. Director Shatner talked to Nimoy, attempting to change his mind, but Nimoy was firm in believing that pain or no pain, brother or no brother, Spock would not betray Captain Kirk. Shatner eventually conceded and had the script adjusted.
