Why did Jonah have such a bad attitude toward the Assyrians? The Scriptures don't tell us why but it is assumed that he hated them because of their wickedness. Nineveh was known as "the bloody city, all full of lies and plunder—no end to the prey" (Nahum 3:1). And the king himself described his people as evil and violent (Jonah 3:8). It could be that Jonah had a personal grudge against them because of something they had done to him or his family. Whatever the reason, he wanted nothing to do with their repentance and subsequent forgiveness by God.
"In 4:2 the prophet tells us himself what his motive was in fleeing to Tarshish. He refused to go because he was afraid God's message would be successful among them. The heart of man naturally prefers judgment upon other men, rather than the manifestation of God's grace and mercy to them." Charles L. Feinberg in The Minor Prophets (Chicago: Moody Press, 1990), 135.
This is the sad truth. Most people would have responded the same way. But God, who is rich in mercy, had a different attitude toward these wicked people. Despite the fact that they had been living in complete opposition to his holy ways, God extended his love toward them and forgave them when they turned from their sin to him. God gave to them what they did not deserve.
This is the the heart of the gospel of Jesus Christ. All of us are undeserving sinners from God's perspective. We have all broken his laws and deserve to be punished by him. "But God shows his love for us in that while we were still sinners, Christ died for us" (Rom. 5:8 ESV). Despite the extent of our sinfulness toward him, God loved us enough to give his Son to die in our place. On the cross, he willingly took upon himself the punishment we deserved. Because the price has already been paid, all those who repent (turn from their sins) and believe that Jesus died for them and rose again are granted forgiveness by God.
In the end it didn't matter what Jonah thought or what we think about it. God is free to show mercy to the vilest of sinners who turns from his sin. And for that we should all be grateful.