John Gill Commentary 1 Samuel 28:5

"And when Saul saw the host of the Philistines, he was afraid, and his heart trembled greatly." — 1 Samuel 28:5 (ASV)

And when Saul saw the host of the Philistines
From the mountains of Gilboa, where he had pitched his camp:

he was afraid, and his heart greatly trembled ;
on sight of the numbers of them, and thinking perhaps of the death of Samuel, and of the loss of David; who was now among the Philistines, and might possibly fight against him, and for the Philistines, of which he might be informed; however, he was not with him, and his conscience might accuse him of various sins he had been guilty of, for which he might fear the Lord would now reckon with him.