“But I have obeyed the voice of Yahweh.” It is unclear here whether Saul is just a stubborn liar who cannot bring himself to be humble before Yahweh and admit his guilt, or whether Saul is so spiritually blind and ungodly by this point that he actually believes he obeyed God. At some level he does know that the word of Yahweh was not fulfilled, because in the next verse he shifts the blame to the people.
“devoted...to destruction.” That is, killed them all. [For more on things “devoted” to Yahweh and devoted to destruction, see commentary on Josh. 6:17].