Haven't we all heard of those who've killed others -- even their own children -- after (they claimed) hearing a voice telling them to do so? Most people would consider these ones to have been insane if they really thought they'd heard such things. On the other hand, some JWs, I would suspect, would think that these murderers may actually have heard demonic voices. Certainly, such behavior fits the profile of what could be considered "demonic".
My question, then, is this: why didn't Abraham suspect that the voice that instructed him to kill his son Isaac was from Satan or a demon? Not only was he being told to perform a demonic act, it was an act that should have seemed clearly inconsistent with God's promise he had received regarding Isaac's future. Did Abraham, perhaps, not believe in Satan or the demons?