Modern ethics are dismayingly utilitarian; the pursuit of happiness for the greatest number defines what is right. Biblical ethics always looks at the nature of thing; an is implies an ought.
Peter Kreeft, in his Three Philosophies of Life writes:
Ancient ethics always dealt with three questions. Modern ethics deals with only one, or at the most, two. The three questions are like the three things a fleet of ships is told by its sailing orders. [The metaphor is from C. S. Lewis.] First, the ships must know how to avoid bumping into each other. This is social ethics, and modern as well as ancient ethicists deal with it. Second, they must know how to stay shipshape and avoid sinking. This is individual ethics, virtues and vices, character- building, and we hear very little about this from our modern ethical philosophies. Third, and most important of all, they must know why the fleet is at sea in the first place . . . I think I know why modern philosophers dare not raise this greatest of questions: because they have no answer to it.’
This explains why NT ethics may differ from OT ethics. OT ethics assume man in Adam, in the flesh, and commands accordingly: NT ethics address the church as man in Christ, in the Spirit, and command accordingly. Responsibilities flow from what is.
This also explains both why it is on the one hand foolish to place NT believers under obligation to OT law and why it is equally foolish to see NT believers as having no obligation or command at all.