Jesus said to love your enemies, to pray for those who spitefully use you. If one of your enemies takes from you, give to him more than he's asked for. If he requires something of you, go the extra mile for him.
But have you read the Psalms?
David is, shall we say, a little less forgiving, loving, and generous. But he does pray for his enemies. In fact, if you read through the Psalms, you will see just how often David prays for his enemies...for God's fiery vengeance upon them.
Heap coals on their heads! Trap them in their own snares! Take their lives before they can take mine! Smite them, O Lord my God. Smite them good!
We read these prayers, and it's easy for us to chalk them up to the idea that the God of the Old Testament was more this kind of God than the God of the New Testament. He's the kind of God who smites our enemies, and even sometimes His own people. He's a God of vengeance, and this kind of thing would have been right up His alley. It must have been, for He calls David "a man after God's own heart" - even the David who prays these prayers.
And maybe that satisfies some of our deep questions (but probably not). But it comes nowhere close to explaining why we are the kind of people who still pray these types of prayers. These types of prayers are far more easy, far more natural for us than the kind Jesus requires. So what do we say about us as a people who grumble to God about our enemies and pray for their demise more than we pray for their redemption? Us as a people who have Christ and still pray these prayers.
Maybe we are not a people after after God's own heart. Maybe we just aren't David. (Most of us have resigned ourselves to this long ago.)
But what if David is us?
What if David's prayer is faithful precisely because it's raw-ly human? What if David is a man after God's own heart because he embraces so much of his humanity? What if we still pray the kind of prayers that David prayed because David prayed the kind of prayers that men pray?
What if David, just like the rest of us, wasn't a man justified by his super-holy works, but a man justified by faith? What if David was a man after God's own heart because of the state of his own heart and not how "right" he was getting it?
What if there's hope/strength/encouragement for us because of David and not in spite of him?
We're all just human, after all.