John Gill Commentary 1 Samuel 18:15

"And when Saul saw that he behaved himself very wisely, he stood in awe of him." — 1 Samuel 18:15 (ASV)

Wherefore when Saul saw that he behaved himself very wisely ,
&c.] So that he could get no advantage against him, and he succeeded and was prosperous in all his enterprises, and was more and more in favour with the people:

he was afraid of him ;
lest the time was drawing near that the kingdom should be rent from him, and given to David.