God's Goodness and Bringing Judgment

In Nahum 1.7, this morning, where Nahum begins by proclaiming God's power and ability to bring judgment upon Nineveh, then suddenly in verse 7, he says the Lord is good. Why the sudden switch to commenting on God's goodness in the middle of a passage about him bringing judgment? I think John Calvin gets it right in his comment on the passage:

First, by saying that God is good, he turns aside whatever might be objected on the ground of extreme severity. There is indeed nothing more peculiar to God than goodness. Now when he is so severe, that the very mention of his name terrifies the whole world, he seems to be in a manner different from himself. Hence the Prophet now shows that whatever he had hitherto said of the dreadful judgment of God, is not inconsistent with his goodness.