Why was Sha'ul's action foolish?
Radak: You saw that whatever I said is fulfilled. Now you did not believe that I would come within the time. You reject me and my prophecy! You did not guard the Mitzvah of Hashem "Lo Samru Es Pi Hashem" (12:4). You were afraid; you removed from your heart Bitachon in Hashem. You thought that you will not save Yisrael if you wait longer.
Malbim: You should have followed the Navi's word simply, without giving explanations and reasoning.
What is "Atah"?
Rashi: It is before you did this.
Why would Sha'ul's kingship be permanent [had he not sinned]?
Rashi citing Zevachim 120a: When one is aggrandized, it is for him and his descendants.
Malbim: This was a test. Had you passed, your reward would be that your kingship would have lasted forever.
The verse implies that if not for this, Sha'ul's kingship would have been permanent. It says "Lo Yasur Shevet mi'Yehudah" (Bereishis 1:15)!
Ralbag: His kingship would have been "Ad Olam", i.e. a long time. We find that 50 years is called "Ad Olam."
The verse does not explicitly say that kingship will be only from Yehudah. Perhaps there would have been a different Perush of the verse. (PF)
Hashem knew from the beginning that Sha'ul will transgress. If not for this, Hashem would not have given Yakov Ru'ach ha'Kodesh to say that verse, or Sha'ul would have been born from Yehudah. (PF)
Why does it say "El Yisrael"?
Radak: This is like Al Yisrael. We find like this - "va'Yamilchehu El ha'Gil'ad" (Shmuel 2 2:9).
