First consider Jehu:
2 Kings 10:30
And the LORD said unto Jehu, Because thou hast done well in executing that which is right in mine eyes, and hast done unto the house of Ahab according to all that was in mine heart, thy children of the fourth generation shall sit on the throne of Israel.God clearly rewards Jehu (quite generously) for killing not just Ahab's reigning son, but also Ahab's entire family. There is a reason that God rewards Jehu for doing this. God had appointed this punishment as a specific punishment that God applied to the families of Jeroboam, Baasha, and Ahab.
Jeroboam, the son of Nebat, who made Israel to Sin:
1 Kings 14:10
Therefore, behold, I will bring evil upon the house of Jeroboam, and will cut off from Jeroboam him that pisseth against the wall, and him that is shut up and left in Israel, and will take away the remnant of the house of Jeroboam, as a man taketh away dung, till it be all gone.Baasha, who departed not from the way of Jereboam
1 Kings 16:1-4
Then the word of the LORD came to Jehu the son of Hanani against Baasha, saying, Forasmuch as I exalted thee out of the dust, and made thee prince over my people Israel; and thou hast walked in the way of Jeroboam, and hast made my people Israel to sin, to provoke me to anger with their sins; behold, I will take away the posterity of Baasha, and the posterity of his house; and will make thy house like the house of Jeroboam the son of Nebat. Him that dieth of Baasha in the city shall the dogs eat; and him that dieth of his in the fields shall the fowls of the air eat.Ahab, who exceeded Jeroboam and Baasha by introducing Baal-worship:
1 Kings 21:20-24
And Ahab said to Elijah, Hast thou found me, O mine enemy? And he answered, I have found thee: because thou hast sold thyself to work evil in the sight of the LORD. Behold, I will bring evil upon thee, and will take away thy posterity, and will cut off from Ahab him that pisseth against the wall, and him that is shut up and left in Israel, and will make thine house like the house of Jeroboam the son of Nebat, and like the house of Baasha the son of Ahijah, for the provocation wherewith thou hast provoked me to anger, and made Israel to sin. And of Jezebel also spake the LORD, saying, The dogs shall eat Jezebel by the wall of Jezreel. Him that dieth of Ahab in the city the dogs shall eat; and him that dieth in the field shall the fowls of the air eat.And Jehu was specifically commanded to execute this judgment against the entire house of Ahab:
2 Kings 9:1-10
And Elisha the prophet called one of the children of the prophets, and said unto him, Gird up thy loins, and take this box of oil in thine hand, and go to Ramothgilead: and when thou comest thither, look out there Jehu the son of Jehoshaphat the son of Nimshi, and go in, and make him arise up from among his brethren, and carry him to an inner chamber; then take the box of oil, and pour it on his head, and say, Thus saith the LORD, I have anointed thee king over Israel. Then open the door, and flee, and tarry not. So the young man, even the young man the prophet, went to Ramothgilead. And when he came, behold, the captains of the host were sitting; and he said, I have an errand to thee, O captain. And Jehu said, Unto which of all us? And he said, To thee, O captain. And he arose, and went into the house; and he poured the oil on his head, and said unto him, Thus saith the LORD God of Israel, I have anointed thee king over the people of the LORD, even over Israel. And thou shalt smite the house of Ahab thy master, that I may avenge the blood of my servants the prophets, and the blood of all the servants of the LORD, at the hand of Jezebel. For the whole house of Ahab shall perish: and I will cut off from Ahab him that pisseth against the wall, and him that is shut up and left in Israel: and I will make the house of Ahab like the house of Jeroboam the son of Nebat, and like the house of Baasha the son of Ahijah: and the dogs shall eat Jezebel in the portion of Jezreel, and there shall be none to bury her. And he opened the door, and fled.So, you see, Jehu obeyed God and killed the whole family of Ahab for the sins of Ahab, and God rewarded Jehu for this obedience. In contrast, consider Amaziah.
Amaziah provides an almost opposite example. Amaziah does not wipe out someone's family, and the reason given is God's law.
2 Kings 14:1-6
In the second year of Joash son of Jehoahaz king of Israel reigned Amaziah the son of Joash king of Judah. He was twenty and five years old when he began to reign, and reigned twenty and nine years in Jerusalem. And his mother's name was Jehoaddan of Jerusalem. And he did that which was right in the sight of the LORD, yet not like David his father: he did according to all things as Joash his father did. Howbeit the high places were not taken away: as yet the people did sacrifice and burnt incense on the high places. And it came to pass, as soon as the kingdom was confirmed in his hand, that he slew his servants which had slain the king his father. But the children of the murderers he slew not: according unto that which is written in the book of the law of Moses, wherein the LORD commanded, saying, The fathers shall not be put to death for the children, nor the children be put to death for the fathers; but every man shall be put to death for his own sin.2 Chronicles 25:1-4
Amaziah was twenty and five years old when he began to reign, and he reigned twenty and nine years in Jerusalem. And his mother's name was Jehoaddan of Jerusalem. And he did that which was right in the sight of the LORD, but not with a perfect heart. Now it came to pass, when the kingdom was established to him, that he slew his servants that had killed the king his father. But he slew not their children, but did as it is written in the law in the book of Moses, where the LORD commanded, saying, The fathers shall not die for the children, neither shall the children die for the fathers, but every man shall die for his own sin.The law referenced is Deuteronomy 24:16: "The fathers shall not be put to death for the children, neither shall the children be put to death for the fathers: every man shall be put to death for his own sin."
There's no explicit praise of Amaziah, but it should be apparent that Amaziah is doing what the Lord commanded.
III. Comparing Amaziah and Jehu
Each of these kings was following the command of the Lord, yet each was doing the opposite. Jehu carried out the judgment of God against Ahab's entire family, whereas Amaziah carried out the judgment of God against only the murderers.
The reason is that Jehu's case was exceptional. Jehu had a specific command from God to kill certain people. Ordinarily, the family of Ahab would not have deserved the civil punishment of death. However, they did deserve the punishment of death in God's sight, and God sentenced them to death and appointed Jehu to be executioner.
Amaziah, on the other hand, had no special command from God. Therefore, he properly limited the civil punishment of death for murder to the murderers themselves.
This difference shows us an interplay of two rules for judgment. One rule is absolute, springing from God's direct revelation. The Canaanites provoked God's anger because of their sins, and God sent the Israelites to wipe them out entirely. However, God did not authorize the Israelites to wipe out all unbelievers at all times.
There was no general civil law against unbelief in Israel. Indeed, the law of God for Israel (both the moral and the civil law) recognized the fact that there would be "strangers" and "aliens" living in the land with the Jews. These are people who are not worshipers of God, but who are nevertheless not under civil sentence of death.
Thus, it is a mistake for people to assume that because the genocide of the Canaanites and the mini-genocide of the Ahabites was commanded, there was a general "intrusion ethic" at play in the civil law. Instead, the civil law stands in contrast to the special execution of God's wrath on Jeroboam, Baasha, and Ahab.
It should be noted, however, that these judgments of God were righteous. While the civil law is just and good - it does not measure up to the standard of God's justice in every respect. Instead, in God's mercy, God limits the civil law. For example, every sin deserves death from God, but God did not appoint in the civil law that every sin would be punished with death.
Likewise, God permitted divorce with few restrictions (in the civil law), even though God hates divorce and even though divorce is (generally) contrary to the moral law. Some might argue that a similar point could made with respect to God's permission of slavery, but we need not argue that divisive point here.
The summation of this comparison is to see that Jehu's actions were commendable only in that they were the execution of God's just judgment upon Ahab and his family. They were not commendable as an example of proper civil law at work. On the other hand, the mercy inherent in Amaziah's restraint in killing only the murderers and not their children is commendable as an example of proper civil law at work.
There is a gospel point to draw from this. We men all deserve death from God, both for the sins of Adam and for the sins of our other fathers. If we were judged simply according to our federal heads according to the flesh, we would all richly deserve death, as did the family of Ahab and Baasha and Jeroboam.
But God extends mercy, both in the civil law, and in allowing us time to repent. If you, dear reader, have not repented of your sins: seize the day. Death is coming. You can try to put it off for a while, but in the end you too are mortal. The only way of escape is to repent of your sins and trust in Christ.
Remember what was written of Ahab:
1 Kings 21:25-29
But there was none like unto Ahab, which did sell himself to work wickedness in the sight of the LORD, whom Jezebel his wife stirred up. And he did very abominably in following idols, according to all things as did the Amorites, whom the LORD cast out before the children of Israel. And it came to pass, when Ahab heard those words, that he rent his clothes, and put sackcloth upon his flesh, and fasted, and lay in sackcloth, and went softly. And the word of the LORD came to Elijah the Tishbite, saying, "Seest thou how Ahab humbleth himself before me? because he humbleth himself before me, I will not bring the evil in his days: but in his son's days will I bring the evil upon his house."If God showed mercy even on the outward repentance of the most wicked Ahab, how much more will he show favor to those who truly repent from their sins wholeheartedly? So, this day repent of your sins and trust in Christ for salvation from the judgment of death (both physical and spiritual), which will otherwise certainly befall you.