Subjectivism and the morally conscientious person's concern to avoid acting wrongly

Subjectivism about moral wrongness is the view that the moral wrongness of an action (if and how wrong that action is) is grounded solely in facts about the agent's mental state at the time of action. Antisubjectivism is the denial of subjectivism. I offer an argument against subjectivism, and for antisubjectivism, based on an examination of the main concern of the morally conscientious person, viz., the concern to avoid acting wrongly.