The nature of Spirits is not so clear cut, they are beings of thought, embodied ideals. An ideal when left to its own design remains pure, unchallenged, but when it is judged it's perception changes. Both Spirits and demons are bound by the nature of the fade. That being that the world will change when you think it must, Justice changed to reflect the only view of Justice Anders could understand, he was bound to it and Anders became more and more convinced that he changed his friend into a single minded shadow of its former self, but not quite a demon. So that's what Justice became, because his nature as a spirit demanded that Anders' s view of him was correct