While Saul makes the right choice to abandon his pursuit of David to go fight the Philistines, he doesn’t give up:
The three thousand men are Saul’s hand-picked special forces:
Doing his Business
להסך את רגליו—literally “to cover his legs”—is a euphemism:
And Saul here demonstrates his modesty; he goes into the depths of the cave to relieve himself. This is in fact a halacha:
The נביא reminds us that, after all, Saul is a צדיק. He may have flaws, but he is known for his self-effacement and modesty. This is fact is his fatal flaw; (שמואל א טו:יז) ויאמר שמואל הלוא אם קטן אתה בעיניך ראש שבטי ישראל אתה.
Cornering the Market
Cutting the corner from Saul’s cloak, while clearly better than killing him outright, was still a strong statement, since it had such symbolic import:
And this explains the text; Rashi is perplexed why David would tell his men חלילה לי מה׳ אם אעשה את הדבר הזה לאדני למשיח ה׳ לשלח ידי בו after he had decided not to hurt him:
Similarly, Radak proposes that there’s a missing bit of dialog that David is responding to:
But I would say that the cutting itself was שלח ידי בו, and that is what David is telling his men. Cutting off the corner of Saul’s garment was in effect claiming the kingship for himself, and David realized that it was not time for that yet.
Ralbag adds another dimension to David’s insistence that כי משיח ה׳ הוא: it would be a very bad precedent for David to attack the rightful king:
And it did work out badly in the end:
Using the singular ואמר להרגך emphasizes the unanimity among David’s men:
And the grammar of תחס עליך is wrong; it literally means “she had pity on you”, not “I had pity on you”. The gemara sees in this David recognizing Saul’s modesty (and, as we said before, his virtue as king, while perhaps hinting at his shortcomings):
And note that David calls Saul אבי:
And this incident proves that David has been innocent all along:
Old Saw
David’s philosophy, that everything will work out for the good if we leave it in ה׳'s hands, is inherent in the proverb he cites.
It’s not our job to play G-d. In any system of justice there is a balance between the risk of convicting the innocent and exculpating the guilty (in medicine, that’s the balance between specificity and sensitivity); we follow the rules even if sometimes the guilty go free. ה׳ will take care of ultimate justice.
And similarly, the fact that someone is destined to be punished, and in fact deserves it, does not absolve you of responsibility:
Saul now realizes the truth: he calls David בני, says he cannot be איבו. He publicly proclaims וקמה בידך ממלכת ישראל, and has David swear to preserve his (Saul’s) descendents, just as Jonathan did. This should be the happy ending of שמואל א. But something happens, and it all falls apart again…