John Gill

And he said to David, thou art more righteous than I,.... By which it appears he thought himself righteous, though David was more so; the righteousness of David was so glaring, that his enemy himself being judge acknowledges it, but will not confess his own wickedness, having no true sense of sin, nor real sorrow for it: for thou hast rewarded me good; in times past, and now; heretofore in killing Goliath, fighting his battles for him against the Philistines, driving the evil spirit from him, by playing on the harp before him, and now by sparing his life, only cutting off the skirt of his garment, when he could with equal ease have cut off his head: whereas I have rewarded thee evil: in seeking to take away his life at various times, by casting a javelin at him more than once, sending messengers to kill him, and hunting after him from place to place, to take him and slay him.