Here come the Leviathan contradictions. I think the reason some people dislike this story is because it has so many inconsistencies.
David reads "the book" and learns that Leviathans must not kill their enemies, so Elizabeth intrinsically knew not to kill Julia. Fine, it saved Julia, at least for the moment.
But, it was the Leviathans themselves (the "true" Leviathans, not the converted like Barnabas) who demanded that Barnabas kill her just the day before!!!