“What have you done?” Samuel asked. And Saul replied, “When I saw that the troops were deserting me, and that you did not come at the appointed time and the Philistines were gathering at Michmash,
Here is, I. Saul's offence in offering sacrifice before Samuel came. Samuel, when he anointed him, had ordered him to tarry for him seven days in Gilgal, promising that, at the end of those days, he would be sure to come to him, and both offer sacrifices for him and direct him what he should do. This we had Sa1 10:8.
Commenting on 1 Samuel 13:8-14
And Samuel said, what hast thou done? This question he put to bring him to a confession of what he had done, otherwise he guessed at it by his countenance; or rather, by the prophetic spirit he was endowed with, he knew it certainly that he had offered the sacrifices without waiting for him: and Saul said because I saw the people were scattered from...
And Samuel said, What hast thou done? And Saul said, Because I saw that the people were scattered from me, and [that] thou camest not within the days appointed, and [that] the Philistines gathered themselves together at Michmash; (h) Though these causes seem sufficient in man's judgment: yet because they had not the word of God, they turned to his destruction.