I believe I understand what you are saying.
My take is that you say person B should be disgusted by murder today, and the normative theory that B subscribes to today holds that B should be disgusted by murder. But tomorrow, B might not be disgusted by murder. So he is the same as the disparaged Humean, except for the two “shoulds” that happened yesterday.
Person A: “B, you subscribe to normative theory X, which holds that you should be disgusted by murder, and that not being disgusted by murder will diminish your flourishing, eudaimonia, and happiness?”
Person B: Yes
Person A: I saw you just now, and you seemed not to be disgusted by the murder that just occurred.
Person B: Oh, well yes. In this case, I chose to diminish my own flourishing, eudaimonia, and happiness.
What is lacking is a proof or demonstration of how subscribing to a particular ethical doctrine prevents or diminishes the immoral act that the ethical doctrine and its supporters imply it prevents or diminishes. If the doctrine doesn’t prevent the immoral act any more so than another doctrine, why should someone change from their current doctrine?
Wombatron, by the way, could you comment on the following post?