Samuel said to Saul, Yahweh sent me to anoint you to be king over his people, over Israel: now therefore listen you to the voice of the words of Yahweh.
Anything "devoted" was to be destroyed utterly and could not be used personally by the victors.  This custom, practiced by all nations was well known to Saul.  Furthermore, his savage murder of the entire priesthood at Nob indicates that there was not a single humanitarian thread in Saul's character.