In the film, Bryce figures that the people in town that were killed by the creature felt that it was in fact Satan that was killing them and so the creature took on that role because that's what the people perceived it as. However, Flyte, Hammond, Jenny and Lisa all figure it's just some primeval creature that enjoyed playing the role of Satan in order to terrify its human prey. In the book, this idea eventually comes to pass but initially, the survivors are convinced they are dealing with a demon because they all got the same feeling of the presence of "pure evil". Eventually Flyte explains that he believes that the creature is Satan. Not the one from the bible however, but the creature that inspired the one in the bible. As every religion and culture around the world all collectively have their own interpretations of evil beings, demons, Satan, etc...Flyte believes that these are all people who have come into contact with "The Ancient Enemy" and lived to tell the story.