I think you missed the point quite badly there. The point is that there is no rationally compelling reason to act on any arbitrary value. You gave the example of punishing murderers, but if every value is equally arbitrary that is no more justifiable than punishing stamp collectors or the left-handed. Having accepted moral subjectivism, you are faced with a choice between acting irrationality or not acting. OTOH, you haven't exactly given moral objectivism a run for its money.