What is the meaning of "Im Gamalti Sholmi Ra"?
Rashi: Did I do to him like his deeds [against me]?!
Radak: One who was b'Shalom with me - did I do evil to him, like he did to me? I was b'Shalom with [Sha'ul], and his son-in-law, and I fought his wars, and he returns evil to me in place of good!
Mesilas Yesharim (Perek 19): One who does Chesed receives Chesed. The more he does, the more he receives. David prayed [in the merit of] this good Midah - he strived to benefit even his enemies.
Malbim: Did I do evil to one who was at peace with me, via gathering soldiers to fight against those who pain me without reason?! It was proper for me to do so (for they wanted to kill me - verse 6)!
What is the meaning of "va'Achaltzah Tzorerai Reikam"?
Rashi: I destroyed his garment, when I cut the corner of his robe. Did I do so to destroy and empty him amidst hatred?! No -it was to inform that I could have killed him, and I did not kill him! "Va'Achaltzah" is an expression of removing garments.
Radak: Not only did I not do evil to [Sha'ul,] who was b'Shalom with me [and did evil to me]. I saved one who afflicted me without reason. I saved Sha'ul for naught 1 , like David saved him from Avishai, who would have killed him, had David not stopped him - "Akenu Na ba'Chanis" (Shmuel I, 26:8). Also when Sha'ul was in the cave, "va'Yshasa David Es Anashav bi'Devarim v'Lo Nesanam Lakum El Sha'ul" (ibid., 24:7). The prefix Vov in va'Achaltzah is like the Vov in "va'Avadecha Ba'u Lishbar Ochel" (Bereishis 42:10), i.e. it is not like you say that we are spies, rather, your servants came to buy food!
I.e. I was not compelled to save him. Alternatively, I saved him for no reason, for shortly after he resumed pursuing me! (PF)
