After Nineveh repents, God relents and does not punish the city. Jonah is angry and tells God that this is why he fled in the opposite direction from Nineveh in the first place–because he knew that God was merciful.
The language that Jonah uses here is traditional language. It occurs famously in Exodus 34: “The Lord, the Lord, a God merciful and gracious, slow to anger, and abounding in The steadfast love (hesed) of God is the assurance of God's loving kindness, faithfulness, and mercy. This assurance rings throughout the Old Testament, and is affirmed more than 120 times in the Psalms. In some hymns of praise the response of the people was likely... More and faithfulness” (v. 6). But Jonah does not use this traditional language to praise God but to accuse God of forgiving a people whom he thought deserved to be judged and punished.