Why would I not? Scripture clearly says "Samuel". If you want to say it was a demon or something show scripture to confirm it please, thanks.
I certainly cannot prove that it wasn't the prophet Samuel. But I find it hard to believe that God would allow a witch to conjure up Samuel's soul. Can you think of *any* instances in the OT where God brought back the soul of a dead holy man to get a message across -- let alone did so through someone who worked with "familiar spirits?" Maybe there are examples -- I just can't think of any.
On the other hand, we have many examples of where demons were commanded by Christ to speak, identify themselves, be silent, etc... So it is possible that God could have forced a demon to speak the words he wanted him to speak, allowing Saul to see what he wanted to see -- Samuel.
But I have another, even more likely explanation from an Orthodox perspective. We have a highly developed sense and tradition of care before the deception of demons. The demons, we are taught, use whatever tool is at their disposal to destroy us because they hate us. One of the Fathers said that the reason that the Legion of demons asked to go into the pigs was that they wanted to destroy something, and that Christ allowed them to do so in order to demonstrate to the people just what the demons would do to everyone if they were allowed by God to have their way.
The message that Saul received through the witch's ministrations condemned him, brought him to despair, and left him with no hope or way of repentance. This is exactly the way of demons -- first they tempt us to sin, then they suggest to us that we are beyond hope or repentance because of that sin.
Saul was certainly left in despair by this encounter, and it contributed to his demise. There *were* other choices available to Saul -- repentance, reconciliation with David, renunciation of the throne, return to the true worship of God. Any of these things he could have done right that very moment, and his life would have been transformed, without the bloodshed and death that actually did ensue on both sides of this civil war.
I have a hard time seeing God speaking through Samuel in a way that did not offer a way out, whereas I do have an easy time seeing a demon (and we know that Satan can appear as an angel of light) speaking thusly in order to ensure that destruction and death would ensue.
Saul did not seek help from God -- he sought help from a witch, and he got what one would expect to find when one looks in those places.
This is *all* my personal speculations. I am unaware of any New Testament references to this passage, or any patristic discussions, although I have certainly encountered the idea that this "Samuel" was actually a demon somewhere, and found it plausible.