“the best of the sheep and the cattle and the fatlings and the lambs and all that was good.” Knowing Saul and the people, there was no intent at this time to sacrifice all these animals to Yahweh, even though that was what Saul said in 1 Samuel 15:15. The fact that Saul spared Agag showed he did not intend to keep the word of Yahweh; he certainly was not going to sacrifice Agag, and what would be the point of keeping him alive? By verse 15 Saul was just trying to make excuses for his disobedience. By that time he was just saying things that might convince others that he intended to fulfill the command of Yahweh even though that was not his real intention.
“devoted to destruction.” In this context, the phrase means “to utterly destroy.” [For more on things “devoted” to Yahweh and devoted to destruction, see commentary on Josh. 6:17].