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David's Mistaken Psalm
This Psalm is a song that David sang to G-d commemorating his salvation from the persecutions of "Kush ben Yemini," which is a reference to King Saul, who was a Benjaminite (see Talmud Moed Katan 16b). Since Saul did not stop persecuting David until the day he died, the occasion would be Saul's death.
David says that it is in G-d that he seeks refuge from his pursuers. If not for G-d's salvation, David's enemies would tear him apart. David says that if he has been treacherous or acted unjustly, then he deserves this fate. If so, he will gladly accept being trampled in the dust. (The implication, of course, is that this is not the case.)
David asks G-d to rise up and direct His anger against those who persecute him. He also asks G-d to give him sufficient strength to carry out His sentences against those enemies. When the nations of the world finally turn to G-d for help, David asks that He refuse them. Let everyone be judged according to their deeds: David according to his own (which were righteous) and the nations according to theirs (whatever they may be).
David hopes for sin to disappear, rather than the sinners (i.e., they should repent and not be sinners any more). Conversely, those who are already righteous should be reinforced in their righteous ways. Only G-d knows a person's thoughts to be able to determine their true motivations. David trusts G-d, his shield, Who saves the pure of heart.
G-d is a righteous Judge and He becomes angered by the evil He sees every day. If a person does not change his evil ways, G-d will slap them down. G-d is "standing by," ready to strike David's antagonists. If a person comes up with an evil plot, he's ultimately digging a trap in which he will himself fall. His evil plans will boomerang back upon him. When this happens, David will thank G-d for his righteousness.
This Psalm starts with the unusual word "shigayon," for which Rashi has several explanations. One is that it is a type of instrument, as in Habakkuk 3:1. Another explanation is that it means a mistake. Rashi offers several explanations, one of which is that David regretted having sung a song of praise to celebrate the downfall of Saul who, for all his faults was still a righteous king.