Repentance Doesn't Obligate God

Although Nineveh had repented, God was in no way obliged to deliver the city because they 'turned from their evil way', but spared them out of pure grace. God acts with grace toward both Nineveh and Jonah, underlining once again the compassionate character so forcefully presented at the beginning of Jonah 4.

--Daniel C. Timmer, A Gracious and Compassionate God: Mission, Salvation and Spirituality in the Book of Jonah (New Studies in Biblical Theology; Downers Grove, IL: InterVarsity, 2011), 133.