The primary purpose of the horror film is to present a deformed and destructive being (DDB), or monster. But not every movie with a DDB is a horror film. If the film's primary purpose is something other than presenting the DDB, then it is not a horror film. Even so, it can include a memorably horrific sequence in which a DDB is presented. An example is the Biblical epic The Ten Commandments (1956).
The primary purpose of The Ten Commandments is to tell the religious story of Moses, not to present a DDB. Nevertheless, at least one sequence in this film is straight out of a horror movie. Moses has been demanding that Pharaoh let the Hebrew slaves go, and Pharaoh has been refusing. So God, through Moses, sends ten plagues against Egypt to persuade Pharaoh. The one that most strikes fear into the Egyptians is the one that works best as a miniature horror movie: the death of all the first-born children of Egypt.
The perpetrator of this plague is the Angel of Death, a luminous green mist that creeps and branches across the night sky as if it were a web being spun. Little by little it descends, and when it reaches the earth, it rolls with a sickly cast along the ground, leaving unharmed those who are not first-born but killing the first-born, whether they are adults or children. It also leaves the Hebrews unharmed; they are preserved by staying indoors and marking their doorways with lamb's blood, according to the instructions of Moses.
The sequence cuts back and forth between the Hebrews inside--who are terrified by the screams of the dying outside, even as the Hebrews partake of their first Passover--and the Egyptians dying in the streets. Not many deaths are shown, and there is no gore, but a couple of deaths are highlighted for emphasis: that of a soldier who is the first-born of the commander of the host, and Pharaoh's own little boy.
The metaphysics of the Angel of Death are not spelled out, but presumably, if this is his job, he is present whenever anybody dies. In that case, what is deformed about his appearance in this film is that he is not felling people at the usual rate or with the usual rationale; instead, by God's order, he is selecting, all in one night, the first-born of an entire nation, as a punishment and a means of persuasion. The persuasion works, and a defeated Pharaoh allows the Hebrews to leave Egypt. So the Angel of Death is not only a deformed and destructive being--a DDB--but a powerful one.
Cecil B. DeMille made many movies besides The Ten Commandments, but as far as I know he never tried his hand at a horror film. Judging by the effectiveness of the Angel of Death sequence, he might have done a good job of it.
Deformed and Destructive Beings: The Purpose of Horror Films