I agree with Matthew Henry's reasoning. It was a devil.
That it could not be the soul of Samuel himself they might easily apprehend when it ascended out of the earth, for the spirit of a man, much more of a good man, goes upward, Eccl 3 21. But, if people will be deceived, it is just with God to say, "Let them be deceived." That the devil, by the divine permission, should be able to personate Samuel is not strange, since he can transform himself into an angel of light! nor is it strange that he should be permitted to do it upon this occasion, that Saul might be driven to despair, by enquiring of the devil, since he would not, in a right manner, enquire of the Lord, by which he might have had comfort.
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I. The spectre, or apparition, personating Samuel, asks why he is sent for (v. 15): Why hast thou disquieted me to bring me up? To us this discovers that it was an evil spirit that personated Samuel; for (as bishop Patrick observes) it is not in the power of witches to disturb the rest of good men and to bring them back into the world when they please; nor would the true Samuel have acknowledged such a power in magical arts: but to Saul this was a proper device of Satan's, to draw veneration from him, to possess him with an opinion of the power of divination, and so to rivet him in the devil's interests.
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III. It is cold comfort which this evil spirit in Samuel's mantle gives to Saul, and is manifestly intended to drive him to despair and self-murder. Had it been the true Samuel, when Saul desired to be told what he should do he would have told him to repent and make his peace with God, and recall David from his banishment, and would then have told him that he might hope in this way to find mercy with God; but, instead of that, he represents his case as helpless and hopeless, serving him as he did Judas, to whom he was first a tempter and then a tormentor, persuading him first to sell his master and then to hang himself.
As to the appeal to the narrative naming Samuel: The Genesis narrative tells us it was a serpent that tempted our first parents, but no one doubts it was the Devil.