God is under no obligation to be merciful.

Then Jehoahaz sought the favor of the LORD, and the LORD listened to him, for he saw the oppression of Israel, how the king of Syria oppressed them.
2 Kings 13:4

God intervened when by terms of the covenant, He did not have to do so. The king of Israel, Jehoahaz, was not a righteous man. The text says that he did evil in the sight of the Lord (2 Kings 13:2). Yet when the Syrian army constantly harassed and attacked Israel, God saw the foreign oppression. When Jehoahaz turned to God in prayer, God answered his requests and turned back the Syrian aggression. The text is not specific as to who Israel's "savior" was (many scholars believe the growing Assyrian threat to the north diverted Syria's resources from Israel), but God did answer a prayer that He was not obligated to respond to in any way.

The reality is that God is gracious and merciful. And He will show His love in ways we do not understand. In the case of Jehoahaz, God brought deliverance from a military power, not because the king was righteous, but because God Himself was gracious. He did it out of His own nature. God always acts out of His own nature. No one tells God what to do or makes Him act any certain way. So when grace comes forward, we are seeing God as He truly is.

I am thankful that God is gracious to sinners. I would not be here if He wasn't that way. And I am so very moved by His mercies shown to me. If not for the redemptive work of Christ for me, I would be no better than a wicked king myself.