Back to Saul and the witch of Endor. If it was a demon, the demon would have refused to comply or would have given better news to Saul even if it was false information. Since the message from the woman came true it had to come from God. It would have to be God speaking through the brain pattern of Samuel.
I disagree with the premise that if it were demon it must have then refused to comply or give better news. Just because demons are fallen angels who rebel against God doesn't mean that they also must all have purely evil intent or be mischievous inherently. This would be akin to stating that a person who doesn't follow the bible or doesn't believe in God (hence rebels to various degrees) is always evil and mischievous but we know that isn't true. demons may have rebelled and may have a certain death awaiting them, but assuming that they then must behave in certain ways because it would make sense I don't think is logical.