Many people complaints about the ending of "El día de la Bestia", most of them are north Americans, used to no-thinking Hollywood plots/endings.
Well, for all of you from me, with love: If you look with attention, you'll notice all over the movie a group of racists who kill homeless and foreigners.
Those are the "devil's henchmen" (one of them the "devil" himself) It's a metaphor, WE (human race) are the "antichrist", there are not supernatural entities such as gods or devils, certain human behaviour is what father Berriortua ends finding as the "devil" Regards