Frailty Fenton wasn't killing "demons" as was his father and brother. He was killing for pure pleasure as he was a demon himself. Fenton didn't see God when he was locked in the cellar. He was actually visited by Satan who was his "god" since he was an evil person. So when he told his father he was visited by god he was not lying. His god told him to kill his father not to stop him from killing "people" but to stop him from killing more of Satan's followers. Fenton tries to draw out his brother to kill him because he is one of God's chosen warriors. It has been said that the Devil will temp and corrupt you when you are at your weakest and most vulnerable. Certainly Fenton was in this state after being locked in the cellar for long. The angel or God perhaps had the ability to foresee this and tried to warn the Dad when the angel told him his son was a demon. Since Fenton had yet to commit any evil deed at that point his Dad did not see anything when he laid his hands on him. Naturally the Dad, since he saw no "truth" when touching his son, thought the angel must be mistaken. Unknowingly, by locking his son in the cellar, the Dad fulfilled the angel's prophecy. In Fenton's weakened state Satan convinced him of his role in life and told him to kill his Father. Again, these are just my thoughts on the movie.
