What does it mean to love our neighbor? Jesus himself summed it up when he said, "do to others what you would have them do to you, for this sums up the Law and the Prophets" (Matt. 7:12). This is of course the Golden Rule. To love your neighbor is to do things that benefit them and not to do things that harm or hurt them.
Love is a matter of intent even more than action. It means that you actually intend others well and act accordingly. It is a matter of the heart. Jesus says, "it is from within, out of a person’s heart, that evil thoughts come" (Mark 7:21). And it is from the heart that truly good thoughts and actions come.
Jesus of course goes one step further. We are not only to love our friends, but our enemies as well (Matt. 5:44). No one is left. We are to love everyone, both our friends and our enemies. We will look at Matthew 5 a number of times in this chapter, but the overall point is summed up in its last paragraph (5:43-48).
You have heard to love your neighbor, but the general sense was also that we hate our enemies (Matt. 5:43). Indeed, if we are honest with ourselves, not a little of the Old Testament certainly feels hateful toward Israel's enemies. One psalmist says of the Babylonians, "Happy is the one who seizes your infants
and dashes them against the rocks" (Ps. 137:9). A whole collection of psalms, the "imprecatory psalms" call on God to destroy the enemies of Israel (e.g., Pss. 7, 58, 109). The prophets Nahum and Obadiah seem to rejoice in the impending doom of Assyria and Edom respectively.
Certainly these are not the only attitudes toward enemies in the Old Testament. The book of Jonah indicts the prophet Jonah for his unwillingness to abandon a prophecy like the book of Nahum. Jonah strikingly extends perhaps the core description of God in the Old Testament to the enemies of Israel: you are a gracious and compassionate God, slow to anger and abounding in love, a God who relents from sending calamity" (Jonah 4:2). 
Jesus says that while they have heard it was okay to love their enemies, they must be complete as their Father in heaven is complete (Matt. 5:48). They must love not only their neighbors, but their enemies as well. Look at how God acts. He sends the precious rain that waters the earth not only to those who serve him but to the unrighteous as well (5:45). He gives the warmth and light of the sun not only on Israel, but on the Romans and other nations as well. So we are to love everyone, including our enemies.
When we are formulating a biblical theology on a topic, the New Testament provides a more complete understanding than the Old. And we don't have to decide between the message of Jesus and the rest of the New Testament, for the New Testament is unanimous in its teaching on loving one's enemies (e.g., Rom. 12:17-21). God will take care of judgment...
 This description of God appears across the Old Testament. Cf. Exod. 34:6; 2 Chron. 30:9; Neh. 9:17; Ps. 86:15, 103:8, 145:8; Joel 2:13; Jonah 4:2)