Madeleine proposed some questions about Old Nick and his motives. Although he is certainly a difficult character to comprehend, here Is what I would guess goes on in his head. Nick clearly still enjoys having sex with ma, otherwise he would have stopped. Presumably, he would find it less enjoyable if he allowed Ma's mental state to deteriorate. I know it sounds very despicable and its possible that old Nick actually cares, but I personally feel that Nick operates under strictly selfish motives.
It does seem to me that he feels some guilt at Jack's perceived death, but I don't think that he is feeling sympathy for Jack or for Ma. I think that Nick has been trying to tell himself that he is not as evil as he could be. He has clearly argued this over and over to Ma, telling her to appreciate how good she has it, and lthough he is not convincing her, I think he convinced himself. but now he thinks he is a murderer in addition to a kidnapper and rapist. Murder, I would argue, is a whole other level. In stead of sympathy, I would hypothesize that Nick recognizes a distinction between and rape and kidnapping on the one hand, and murder on the other, and that he feels some moral qualms at having taken this extra step. This would also explain why he was so angry at having been deceived, instead of simply being scared. Although the fear prevailed in the end, Jack said that Nick had "the angriest face I ever saw." If someone tricked you into thinking you were a murderer, thereby outting you into danger, you would be pretty upset at them, more so than if you had simply been tricked and put into danger.
Anyway all of this is hypothesis. there is no way we can actually comprehend how Old Nick's mind functions, especially because none of us share his motives (I hope).
