Nineveh was the capital city of Assyria. Assyria was known for being a bloodthirsty, vicious nation. They were known for destroying everything in their path. They were a well-trained army that took captives into slavery or outright slaughtered them. They impaled the vanquished leaders outside the gates of the cities. They had been a thorn in the side of the Israelites for decades. Now God is telling Jonah to bring salvation and forgiveness to a hated enemy.
Jonah was furious because after he did as God commanded, the city repented and received forgiveness. Now they would not receive punishment for the thousands of Israelites they had killed and tortured. Jonah wanted them to pay for the pain and horror they had created.
God's love is without prejudice. He doesn't take into account our past sins and failures. He sees our hearts and our true motivation in accepting His free gift of salvation. When we are willing to forgive our worst enemies, He is willing to forgive our sins. (Matt 6:12)
Sin is the same across the board. In God's eyes, when we break His laws, we are guilty. Paul's sin of persecuting the early church was wiped out just the same as my sins were. No matter what name we put on sin, it is still sin. Whether it is gossip, murder or unfaithfulness, God sees no difference. He sent His Son to pay our debt and give us eternal joy and forgiveness.
God's love is so deep and wide and high, it is beyond our understanding. He looks at His people with acceptance and delight when they turn from their sin and seek Him. We are encouraged to study to show ourselves approved. When we learn the character of God, we fall deeper in love with Him. He desires all to come to Him. He can even use us to achieve His goal.