Absalom instructs his servants to kill Amnon for what he did to Tamar. This is not justice but vengeance. Vengeance is forbidden, and we should leave judgement with God because we too deserve judgement. A false story is reported to David but Jonadab, who had been a venomous counsellor to Amnon, brings the truth; perhaps he was in on it and advised Absalom? If someone gives us advice contrary to the word of God, they are not a true counsellor or friend. David is spent but his heart goes out to Absalom. God is not spent by our sin. There is victorious mercy in Christ.