1)
Why does it say "v'Atah Shma l'Kol Divrei Hashem"?
1.
Rashi: Once you were foolish. Now, be careful!
2.
Radak: Since you were anointed to be king, you must do what He commands you, i.e. to fight Amalek. Yisrael were commanded three Mitzvos when they entered Eretz Yisrael - to appoint a king, wipe out the seed of Amalek, and build the Beis ha'Mikdash (Sanhedrin 20b). About this king (Sha'ul) it says "v'Yarom me'Agag Malko" (Bamidbar 24:7).
3.
Malbim: This Mitzvah [to wipe out Amalek] is a consequence of being anointed king. Therefore, it is a test of keeping your kingship, so heed Hashem's word!
