My interpretation was that David was subconsciously trying to save Auntie from a "punishment." You'll note that she told him that she wasn't thinking of the book, just of Julia and their friendship and he was the one who insisted she was a "true Leviathan."
Thats a good way of interpeting it Luciaphil, I hadnt really thought of looking at it that way before but it makes sense to me now. We can see the look of distress sometimes on their faces when faced with an order as to hurt or kill someone that they've especially known for a long time now.
David looked rather upset when Alexander was arguing with Amy over the picture and he did try many times to talk Amy into just giving Alex the photo back. With his Aunt Elizabeth I could see the same distressful look when she first told him about their plans for Julia. You can see that they still have their own beliefs and feelings still intact, which is good thank goodness, but at the same time they're trying to be good followers of the leviathans, who it seems have no feelings at all.
Deb