Both Jonah and Nahum address Nineveh, an ancient and violent enemy of the Hebrew people. They embody surprisingly different theological and ethical stances towards their enemies.
For instance, Nahum opens up with a powerful declaration of divine wrath: A jealous and avenging God is the LORD, the LORD is avenging and wrathful; the LORD takes vengeance on his adversaries and rages against his enemies. The LORD is slow to anger but great in power, and the LORD will by no means clear the guilty (Nah 1.2-3). Later on he says, Who can stand before his indignation? Who can endure the heat of his anger? His wrath is poured out like fire, and by him the rocks are broken into pieces . . . no adversary will rise up twice (vv. 6, 9).
This anger is directed at the hated enemy of Nineveh, the Hebrews' violent neighbors to the north. God has finally come out in judgment against them, and has declared to them, Your name shall be perpetuated no longer; from the house of your gods I will cut off the carved image and the cast image. I will make your grace, for you are worthless (v. 14). This act of catastrophic judgment against an oppressive force brings rejoicing among the victims: There is no assuaging your hurt, your wound is mortal. All who hear the news about you clap their hands over you. For who has ever escaped your endless cruelty? (3.19)
Clapping over the news of the death of Nineveh -- that sounds like something Jonah would have done, had things gone the way he'd wished. We know his story: he was sent to preach to the Ninevites that God would destroy them in forty days; but hearing the message, they repented and God took pity on them. Jonah protests: O LORD! Is this not what I said while I was still in my own country? That is why I fled to Tarshish at the beginning; for I knew that you are a gracious God and merciful, slow to anger, and abounding in steadfast love, and ready to relent from punishing (Jon 4.2). In the universe of Jonah, it's not so much God's wrath as his forgiveness and mercy that are to be feared; God may end up forgiving your enemies, whose deaths you may desire with great fervor.
Both Nahum and Jonah make reference to the self-description formula from Exod 34.6: The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness, keeping steadfast love for the thousandth generation, forgiving iniquity and transgression and sin, yet by no means clearing the guilty, but visiting the iniquity of the parents upon the children, and the children's children, to the third and fourth generation. But they make use of opposite ends thereof: Jonah cites the gracious end, and Nahum makes use of the end dealing with the punishment of the guilty.
This raises the question: is there a perfect symmetry in the self-description here? Is either side as true to God's nature and desires and will as the other? Is it all the same to God to be merciful to a repentant sinner, or to destroy the unrepentant sinner? Does he have any preference at all?
Some persons think and theologize in such a manner as to suppose an equality between the two extremes. Augustinians, for instance, who argue that God creates some persons for the sake of displaying his mercy to them, whereas he creates others for the sake of showing his just hatred of sin in them, presuppose an equality in the divine nature between these two extremes. God is a paradoxical mix of both worlds, an unfathomably generous God who is willing to undergo death himself for the sake of his chosen ones, but also profoundly strict, imposing an unending punishment of torment upon sinners. That one and the same being could equally embody both extremes seems to me problematic, however. If God is capable of damning sinners in hell forever, and it is all the same to him to save a person or to damn him, then what confidence can you have that you are chosen? What confidence can you have that really, God loves you? Barth's argument in CD II/2 about the two-fold darkness in the traditional position on predestination is a good one.
For other persons, however, there is an important asymmetry between the two ways God is manifested. Though he punishes, though he can be strict and even severe, yet deeper, underneath the surface of any apparent display of malevolence is actually a mysterious love, a commitment to every creature whatsoever. Why think such a thing? There may not be any reason to do so, except for the fact of God's self-revelation in Christ, the propitiation for our sins, and not for ours only, but for the sins of the whole world (1 John 2.2). In Jesus Christ the Creator dies for the creature, setting aside wrath and justice and judgment in favor of mercy. This suggests to us that God is love (1 John 4.8), a much more succinct definition of God than we find in Exodus, and the necessary filter by which to interpret the latter. We find and meet this love in many different ways, some of them pleasant and others of them not so pleasant, but at the end of the day, it is love behind all appearances. Thus it is not all the same for God to save a person or to damn him: Do I take any pleasure in the death of the wicked? declares the Sovereign Lord. Rather, am I not pleased when they turn from their ways and live? (Ezek 18.23) Likewise Jeremiah writes: For no one is cast off by the Lord forever. Though he brings grief, he will show compassion, so great is his unfailing love. For he does not willingly bring affliction or grief to anyone (Lam 3.31-3).
But we shouldn't for this reason try to downplay the reality of divine wrath, such as Nahum appreciates. It is perfectly natural and understandable that the victims of violence and oppression and abuse, such as the Hebrews were, should rejoice when all of that is put to an end. It is perfectly appropriate and required at times that God respond to sin with anger, demanding an immediate end to the hurt of his creatures. Yet we must always remember that God is not like us to give up on the depraved. The destruction of Nineveh was not his final dealing with those people. He is love, and we are not; even in moments of wrath and fury, we must remember love as well.