John Gill

Then said Abishai to David,.... Seeing Saul fast asleep, and a spear so near him: God hath delivered thine enemy into thine hand this day: or at this time, properly it was night: now therefore let me smite him, I pray thee, with the spear; with Saul's own spear, which was stuck in the ground at his bolster. He remembered that David would not put forth his hand to stay him before, when he had an opportunity; and since now another offered, he did not move it to him to do it, but begged leave to do it himself; which he might think would be granted, since there was such a remarkable hand of Providence in it, which seemed to direct to such a step: even to the earth at once, and I will not smite him the second time; signifying, that he would give such a home blow or thrust, that the spear should pierce through him, and fasten him to the ground, that there would be no need to repeat it.