Glen H. Stassen of Fuller Seminary writes that the function of the Sermon on the Mount was to reinterpret OT Law by changing the negative prohibition of the Law into positive activity. So what exactly does this mean…? Traditionally interpreters have tended to argue that Jesus takes these laws and broadens their scope to say "not only should you not commit murder (easy), but even those who have anger in their hearts are commiting a form of murder (harder)." Then each teaching is followed by an illustration of this new concept of murder.
Instead of this, Stassen proposes the idea that the illustration itself is the ultimate reinterpretation of the Law. So for Jesus, following the Law is not merely avoiding murder (easy), nor is it avoiding anger (harder), but it is the positive, outward act of making peace (hardest) that is the ultimate responsibility of believers if they intentionally desire to obey the law.
The article forcefully demonstrates how each of the 14 illustrations enforces this concept of authentic obedience to the Law as positive, ongoing activity.