The historical and religious context:

1. While approaching the land, the Amalekites had attacked them, preying on the weak. God had said that they would be destroyed. Now, probably partly as a test for their first king (he failed, didn’t eradicate them), God said, get on and do it.

2. God had promised the land to Abraham and his descendants, but said they’d only get it in four hundred years’ time, because “the iniquity of the Amorites is not yet complete”—they still had time to choose God’s ways. Only once they were irredeemable were they to be destroyed.