Beyond the fact that the plot of the Horus Heresy series requires a certain ending to facilitate the context for Warhammer 40K, I don't understand why it was so difficult to see the demons as merely xenos of the Warp. Was there not one person who counseled this idea instead of merely saying "Demons don't exist!" Again we have a problem with definition which would be solved if people just asked "What do you mean by ____?" In a world of psychers and the Warp, the Imperial Truth is valid.