Why does it say "Atah" again?
Malbim: Verse 13 taught that he will not receive permanent kingship as a reward for guarding the command. Our verse teaches the punishment for violating it.
Why did Sha'ul lose kingship due to this?
Rashi: It says "l'Vilti Sur Min ha'Mitzvah? Lema'an Ya'arich Yamim?" (Devarim 17:20) - if he deviates, even from a Navi's command, he will not endure as king.
Malbim: You thought, unlike my words, that your kingship will be based on nature and war strategy. Therefore, you did not wait. 1 This was foolish. Had you cast your concerns on Him, your kingship would pertain to Him, and not yourself, and it would have lasted forever.
Even though Sha'ul merely offered a Korban, which shows reliance on Hashem, he did so amidst concern that people are leaving. (PF)
Did Shmuel know who will receive kingship?
Radak: No, he just knew via Ru'ach Nevu'ah that someone else will receive it.
