From Richard B. Hays, Moral Vision of the New Testament: A Contemporary Introduction to New Testament Ethics (pp.336-337):

“…the New Testament’s witness is finally normative.  If irreconcilable tensions exist between the moral vision of the New Testament and that of particular Old Testament texts, the New Testament vision trumps the Old Testament.  Just as the New Testament texts render judgments superseding the Old Testament requirements of circumcision and dietary laws, just as the New Testament’s forbidding of divorce supersedes the Old Testament’s permission of it, so also Jesus’ explicit teaching and example of nonviolence reshapes our understanding of God and of the covenant community in such a way that killing enemies is no longer a justifiable option.  The sixth antithesis of the Sermon on the Mount marks the hermeneutical watershed.  As we have noted, the Old Testament distinguishes the obligation of loving the neighbor (that is, the fellow Israelite) from the response to enemies: ‘[B]ut I say to you, Love your enemies and pray for those who persecute you, so that you may be children of your Father in heaven.’  Once that word has been spoken to us and perfectly embodied in the story of Jesus’ life and death, we cannot appeal back to Samuel as a counterexample to Jesus.  Everything is changed by the cross and resurrection.  We now live in a situation in which we confess that ‘in Christ God was reconciling the world to himself, not counting their trespasses against them, and entrusting the message of reconciliation to us’ (2 Cor. 5:19).  Those who have been entrusted with such a message will read the Old Testament in such a way that its portrayals of God’s mercy and eschatological restoration of the world will take precedence over its stories of justified violence.”