Rejoicing over the Fall of Your Enemies

How do we reconcile the seemingly contradictory instructions of Scripture concerning our enemies?

Do not rejoice when your enemy falls, and let not your heart be glad when he stumbles, lest the LORD see it and be displeased, and turn away his anger from him.
Proverbs 24:17-18

I will give peace in the land, and you shall lie down, and none shall make you afraid. And I will remove harmful beasts from the land, and the sword shall not go through your land. You shall chase your enemies, and they shall fall before you by the sword. Five of you shall chase a hundred, and a hundred of you shall chase ten thousand, and your enemies shall fall before you by the sword.
Leviticus 26:6-8

Then they returned, every man of Judah and Jerusalem, and Jehoshaphat at their head, returning to Jerusalem with joy, for the LORD had made them rejoice over their enemies. They came to Jerusalem with harps and lyres and trumpets, to the house of the LORD. And the fear of God came on all the kingdoms of the countries when they heard that the LORD had fought against the enemies of Israel.
2 Chronicles 20:27-29

If you meet your enemy's ox or his donkey going astray, you shall bring it back to him. If you see the donkey of one who hates you lying down under its burden, you shall refrain from leaving him with it; you shall rescue it with him.
Exodus 23:4-5

You shall not hate your brother in your heart, but you shall reason frankly with your neighbor, lest you incur sin because of him. You shall not take vengeance or bear a grudge against the sons of your own people, but you shall love your neighbor as yourself: I am the LORD.
Leviticus 19:17-18

You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ But I say to you, Love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven. For he makes his sun rise on the evil and on the good, and sends rain on the just and on the unjust.
Matthew 5:43-45

From Jay Carper at American Torah, http://www.AmericanTorah.com