1 Samuel 20:15

but also thou shalt not cut off thy kindness from my house forever, kindness of the nature shown by Jehovah, the covenant God, to His people; no, not when the Lord hath cut off the enemies of David, every one, from the face of the earth. Although Jonathan was a member of Saul's house, who was an enemy of David, he himself was united with David by the bonds of the truest friendship, and therefore wanted both himself and his children spared when David's time of revenge would come.