Matthew Henry on 1 Samuel 28:1-6

David could not refuse Achish without danger. If he promised assistance, and then stood neuter, or went over to the Israelites, he would behave with ingratitude and treachery. If he fought against Israel, he would sin greatly. It seemed impossible that he should get out of this difficulty with a clear conscience; but his evasive answer, intended to gain time, was not consistent with the character of an Israelite indeed. Troubles are terrors to the children of disobedience. In his distress, Saul inquired of the Lord. He did not seek in faith, but with a double, unstable mind. Saul had put the law in force against those that had familiar spirits, Ex 22:18. Many seem zealous against, sin, when they are any way hurt by it, who have no concern for the glory of God, nor any dislike of sin as sin. Many seem enemies to sin in others, while they indulge it in themselves. Saul will drive the devil out of his kingdom, yet harbours him in his heart by envy and malice. How foolish to consult those whom, according to God's law, he had endeavoured to root out!