1)
Why will Hashem destroy them simply becsuse they will not destroy the Cana'anim?
1.
Seforno: Because if Yisrael do not destroy the Cana'anim, they will inevitably end up worshipping idols like them.
2.
Rashbam: Because I commanded them "Lo Sechayeh Kol Neshamah" and they let them off the hook and allowed them to live, Hashem will not let them live.
