Doug Moo gives this thought-provoking perspective on the use of the law in evangelism:
. . . the popular notion that the Mosaic law should be preached as a preparation for the gospel, revealing sin and one’s need of salvation, has slim biblical support. None of the examples of evangelistic preaching in the New Testament uses the law in this way. The closest is Jesus’ encounter with the rich young man . . .. Here Jesus’ citations of the commandments may have the purpose of revealing to this young man his need of the gospel. However, it was not the commandments of the Mosaic law, but Jesus’ “gospel” demand to follow him that drove the young man to despair. Moreover, Jesus cited the Mosaic law because he could assume the applicability of that law this Jewish man. We must reiterate at this point the importance of keeping the New Testament salvation-historical perspective in view and of exercising caution in elevating what was true for Jews under old covenant with its Mosaic law to the status of a general theological principle.
From “The Law of Christ as the Fulfillment of the Law of Moses” in Five Views on Law and Gospel (Grand Rapids: Zondervan, 1996)., 340.