To whom did David swear?
Rashi (like R. Yochanan in Vayikra Rabah 23:11): He swore to his Yetzer ha'Ra.
Rashi (like Reish Lakish in Vayikra Rabah 23:11): He swore to Avishai - if you strike him, I will mix your blood with his!
Radak: He swore to Avishai - do not be concerned lest Sha'ul live a long time. Shmuel told David so through prophecy, so he could swear about this.
What is the meaning of "Hashem Yigafenu"?
Rashi: He will bring his death close.
Radak: He will make him sick before [his time to die], due to his Aveirah. This is like "va'Yigof Hashem Es Naval" (25:38) - he will die through Hashem, and not through me.
Why did he mention Hashem twice?
Rashi: He swore twice - that you (Avishai) not strike him, and if you will, I will mix your blood with his!
Why did he mention "Yomo Yavo"?
Radak: This was to silence (calm) Avishai. Even if Sha'ul will die at his destined time, since Hashem despised him from king and anointed me, surely he cannot live long. Or, perhaps Shmuel told David so through prophecy.
Malbim: He told Avishai, do not think that since I was anointed, you may kill him to establish my kingship. Sha'ul's death will come early, or by itself in its time.
Why did he mention dying in war?
Radak: Also this is before his destined time. It is due to a greater Aveirah than one who dies early through illness. Through merits, one can live longer than his destined time. Tana'im argue about this. 1
Radak citing Yevamos 49b: R. Akiva says, "I will fulfill the number of your days" (Shemos 23:26) are Shnei Doros, i.e. "Yemei Shenoseinu Shiv'im Shanah" (Tehilim 90:10). If he merits, his years are fulfilled. If not, they are reduced.
