But, especially Bruno, thinking he can kill Jeb and then take over as the Leader!! Even Nicholas has said that Jeb is the one indispensable. He's the creature that grew from the box. Bruno was so anxious to serve him when he arrived. This isn't just some ordinary band of "bad guys" where someone might plot a mutiny and want to take over. Jeb is their supernatural creature leader. They all have blind obediance to him. Not only is it unthinkable that Bruno could plan such an attack on him, but clearly it would be impossible for him to take over.
I got the impression that followers were promised that they would become leviathans, too. However when Barnabas was being lectured by Oberon, he indicated they were just using the humans. Maybe, Bruno was lied to like everyone else. Barnabas said to Oberon that people would not follow them if they were asked to do things that they could not do. Barnabas was unable to murder Julia just because the Lung Brat told him to. Elizabeth had trouble with those demands, too. Bruno seemed to want the power and began to chaf at the demands of a petulant, childish leader. Elizabeth was beginning to resent being ordered about by Jeb. She told him this was still her house and he sent her to her room. It just goes to show that there was a leadership vacuum because of Jeb's lack of charisma.
The story also mentions that during the ceremony Carolyn is supposed to change. Jeb even believed it but I think that was also a lie. I think it was supposed to be like Rosemary's Baby. Rosemary did not quite recall that she was raped by the Devil. I think that would have been Carolyn's experience, too.