The prophet came to the King of Israel and said, "Strengthen your position and see what must be done, because next spring the king of Aram will attack you again."--1Kings 20:22
I have the hardest time understanding mercy. By its very nature, mercy implies the principle of subjection: I cannot show mercy unless the other party is under my power. I know that You are merciful, gracious and compassionate, and slow to anger (Joel 2:13). I know You save men purely on the basis of your mercy (Titus 3:5) and you delight in doing so (Micah 7:8). I also know that You command me to demonstrate mercy to others (Micah 6:8, Luke 6:36, Matthew 5:7). But, then, there are those times....
Like in 1Kings 20, when you commanded Israel's King Ahab to attack the blasphemous King of Aram and he did, but then showed mercy to him at the end. Ahab recognized the attacking king of Aram as his brother, sympathized with him, and You punished him for it. And You certainly did not show mercy on the Pharisees in Matthew 23. You called them every name in the book, Your book, condemning them for their own blasphemous behavior. You did not immediately destroy the pharisees, but the Romans did about thirty years later, when they destroyed the temple, leaving only the rabbis. These two instances of withheld mercy tie the Old Testament to the New Testament, bridging the old law and the new, the supremacy of the law to the gospel of grace. I have to find some common ground here.
I think your lesson in the application of mercy lies here: Both the King of Aram and the pharisees denied God's power: the King in words, the pharisees in action. That was bad enough, but they had something else in common. They also both held positions whereby they each wielded authority over others and, if they went unchecked, would continue to harm them. The Tanach, my Hebrew Old Testament, says this: "Mercy to the evil is in itself a manifestation of cruelty, for the surviving evildoer will cause others to suffer." This is the principle you define in 1Kings 20:22 above when Your prophet tells Ahab that, if he doesn't eradicate the evil before him, it will return.
If this idea had remained confined to Old Testament judgement, I might not consider it so compelling, but it didn't. You continued to demonstrate it in the New Testament when you specifically accuse and condemn the pharisees for corrupting those under their care (Matt 23:15). Like Ahab, I also recognize my brotherhood with some who blaspheme your Name. Mercy, however, belongs to You and my sympathy and feelings for someone against whom You send me to war cannot supersede Your clear instruction. Mercy comes from and belongs to you.
I will have mercy on whom I will have mercy, and I will have compassion on whom I have compassion."--Exodus 33:19 and Romans 9:15.