Jamieson, Fausset and Brown on 1 Samuel 18:17

17. Saul said to David, Behold my elder daughter Merab, her will I give thee to wife--Though bound to this already [1Sa 17:25], he had found it convenient to forget his former promise. He now holds it out as a new offer, which would tempt David to give additional proofs of his valor. But the fickle and perfidious monarch broke his pledge at the time when the marriage was on the eve of being celebrated, and bestowed Merab on another man (see on 2Sa 21:8); an indignity as well as a wrong, which was calculated deeply to wound the feelings and provoke the resentment of David. Perhaps it was intended to do so, that advantage might be taken of his indiscretion. But David was preserved from this snare.