But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Note 22 at Mt 5:44: The thrust of Jesus' statements in Mt 5:38-48 is simply that we are not to take matters into our own hands and defend ourselves. "Vengeance is mine; I will repay, saith the Lord" (Ro 12:19; see also De 32:35-36 and Heb 10:30). Striving to vindicate self actually shows a lack of faith in God keeping this promise. It also indicates spiritual "nearsightedness," which is only looking at the present moment instead of seeing things in view of eternity.
One of the best examples of this was David in 1Sa 24 and 26. David had many opportunities to avenge himself against King Saul but refused to do it. He trusted God to avenge and promote him (Ps 75:6-7). As a result, all of God's promises to David came to pass, and in his old age, God showed David mercy (2Sa 15-18) even as David had shown mercy to Saul.
Even as Christ did not come to condemn the world (Joh 3:17) and was not holding people's sins against them (2Co 5:19), even so, we have been given the same ministry of reconciliation (2Co 5:19-20). For those who do not receive the love we extend to them but rather take advantage of us because of our "turning the other cheek" (Mt 5:39), God will repay them (Re 13:10 and 14:9-12).