King David is furious at hearing this (2 Samuel 13:21), but rather than obey the law and banish Amnon as written in Lev 20:17, he instead chooses to sit back and do absolutely nothing at all. Why? Because there were no witnesses and every crime must be settled by a corroboration of the truth rather than a single source? Unlikely, the royal investigation uncovered Jonadab’s involvement at a minimum, and Tamar would easily have been able to provide the necessary circumstantial evidence to settle the matter. I think it’s more likely that David did nothing because he was too ashamed from his recent affair with Uriah’s wife to come down hard on his wayward son. He himself had been shown grace after committing adultery with Bathsheeba, so who was he to pronounce a harsh sentence upon a lesser crime? Thus, instead of being obedient to the law, David gives grace. But God is not mocked, and Deut 27:22 pronounced a curse upon Amnon for his behavior which eventually resulted in his brother Absalom killing him, despite David's grace.
If this was indeed the motivation for David’s behavior (and it seems likely it was) then the lesson is that grace is not to be used by man. That’s a tough thing to come to grips with, and yet, as C.S. Lewis has observed before, this is also the kind of thing that is too hot to hold and yet not too hot to drink. In this way it is similar to God’s sovereignty, or human depravity. In the abstract these doctrines are cruel and painful, and we by nature recoil from them, but when they're accepted and internalized they liberate us with the glorious freedom of God’s God-ness, and we'll defend them ferociously as bedrock truth.
On the surface the idea that grace isn't ours to disperse sounds not only wrong but completely unbiblical. As it is written, "love keeps no records of wrongs" and "blessed are the merciful, for they shall be shown mercy." But after some consideration it becomes evident there is no contradiction between God's command to show grace and the warning about doing it on your own terms. Those verses come after 2 Samuel, which means we ought to first learn the horrible lesson that even showing something as maximally wonderful as grace to our own flesh and blood can condemn them to eternal perdition when it is done in opposition to Scripture.
We have to accept the truth that the requirements for dispensing grace are higher than any man has any access to, for man is but a creature, and a fallen one at that. When God gave grace to David it resulted in his repentance because God is all wise, and knows the ends from the beginning. But because David isn’t omniscient the grace shown to Amnon only sped him on towards his inevitable destruction. Grace didn’t cure the sin, it only fed it and caused it to metastasize until the sin was fully grown, at which point it brought forth death. The problem is that men are so thoroughly ruined and infected by sin that nothing they touch is immune from corruption—nothing. Not even something so lofty and pure as forgiveness can escape the misery brought on by our sinfulness, for in our hands even God’s means of our salvation becomes lethal. In this grace may be likened to the Ring of Power from the Lord of the Rings which was beyond the power of man to control. Recall the scene at the council of Elrond when Aragorn’s explains why they can’t just put on the ring and destroy Sauron with it: “You cannot wield it! None of us can!”
Or if you would prefer a less fictitious comparison, grace is as out of reach for us to do good with as is communism, or legalized theft.
Grace belongs to God, and to God alone. We are not strong enough, or wise enough, or smart enough, or moral enough to use it properly. If we try we will instead only bring down devastation. We are weak, our appetites are fallen, and we are infested with sin, making us as not to be trusted with such a royal tool as greedy Isildur himself was with the ring. (Perhaps this is why Jesus did not entrust Himself to us, knowing what is in the heart of a man, John 2:24). If this is still too difficult to accept then consider that it's the same lesson as why we are not permitted to make life or death decisions on our own; why we’re not allowed to commit suicide, for example.
Take as an example King Saul, who when he realized the Philistines were about to capture and humiliate him, chose instead to fall on his sword and take his own life rather than be paraded about as a captive while the uncircumcised crowed their victory over him. Saul’s fear of pain drives him to do an irreversibly foolish thing, and so he dies a lost man. But what if God wanted Saul to be captured and humiliated? What if living in a cage and being poked at was the one thing that would drive him to repentance like it did the wicked king Manasseh many years later? What if by killing himself to avoid an imagined pain Saul deprived himself of reconciliation with the One True God? Instead of having courage Saul swallowed the lie that he had the authority to make life and death decisions over himself, and played at being God to his destruction. We must therefore learn from Saul that God puts boundaries on things not because He hates us, but because He knows what’s best for us. Just as parents don't ask their crawling babies to attend to the fire heating the house, or recommend they use a chainsaw to cut down a tree, God doesn't want us to get in over our heads by using His tools. He is God; we are not.
The lesson then is that it is our duty to trust God and obey the law, and it’s His job to be God. God has not given us sufficient light to see further than His commandments, and it is only the law which acts as a light to our feet and a guide to our path, so it is only by the light of the law that we must live. When God says to give grace, we must give grace. When He says to discipline, we must discipline. Where the law directs us, we must go. When the laws says to do, we must do. And then, once we’ve learned obedience in all things, we will not have a problem loving our brother or showing mercy as instructed. We will play the notes as they are written down for us and not insist that with the improvisation of a sledgehammer to the keys we can improve the score.
In light of the loving instructions showered down upon us for our good we must learn the lesson of Saul and David and lay down the notion that we are like God, able to fathom His ways. We ought to instead accept the daily provision He has set out for us and be grateful that His eye is on us for our good. As the preacher has taught us: “Let us hear the conclusion of the whole matter: Fear God and keep His commandments, for this is the whole duty of man.” Ecc 12:13