Jesus is giving us a way by which we can keep from being defined by those who act unjustly toward us. When we respond to violence with violence, whether it be physical, verbal or attitudinal, we legitimize the violence of out enemy and sink to his level. When we instead respond unexpectedly- offering our other cheek and going the second mile - we reveal, even as we expose the injustice of his actions, that our nemesis doesn't have the power to define us by those actions. In this sense we serve our enemy, for manifesting God's love and exposing evil (the two always go hand in hand) open up the possibility that he will repent and be transformed.
Peter addressed this point when he spoke to a congregation about to undergo unjust persecution. "When [Jesus] was abused" Peter said, "he did not return abuse; when he suffered, he did not threaten; but he entrusted himself to the one who judges justly" (1 Peter 2:23). So when we are persecuted we are not to resort to violence (as Peter himself had done in the Garden!), but we are to "sanctify Christ as Lord" in our "hearts." IN this way, he continues, we "put those who abuse [us] ... to shame" (1 Peter 3:16). Our refusal to sink to the level of our enemy opens up the possibility that the enemy will see the injustice of his treatment and perhaps be freed from his dehumanizing mindset.
- Gregory A. Boyd The Myth of A Christian Nation