Albert Barnes Commentary 1 Samuel 15:3

"Now go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass." — 1 Samuel 15:3 (ASV)

Utterly destroy - Rather, “devote to destruction” (see the note on Leviticus 27:28). When a city or people were thus made cherem, everything living was to be destroyed, and no part of the spoil was to fall to the conquerors (Compare 1 Samuel 15:21). The valuables were put into the sacred treasury.