He took Agag the king of the Amalekites alive but devoted to destruction all the people with the mouth of the sword. Bible

“devoted to destruction.” Saul and his army killed all the people. [For more on things “devoted” to Yahweh and devoted to destruction, see commentary on Josh. 6:17].

“the mouth of the sword.” Used to show great destruction, as if the sword was eating its victims (see commentary on Josh. 6:21).

Commentary for: 1 Samuel 15:8