It wasn't that “Exodus” God & Kings” was that bad of a movie.
It is more that it could have been better.
The narrative did succeed in creating dramatic interpersonal tension between Moses and Pharaoh by emphasizing the intertwined family relationships of the two characters.
While the film strives to acknowledge in its own way the broad strokes of the Biblical saga, the producers could have done a better job of honoring and adhering to the specifics of the text.
For example, though Aaron is given a supporting role in the story, he tends to look on as Moses haggles with God.
The audience is left to wonder if deity is actually communicating with the prophet or merely a delusion initially induced by a cranial trauma.
Given that the director was Ridley Scott, for all we know the entity manifesting itself in the form of a young boy claiming to be God could have been related to the creatures from the Alien films and alluded to in Prometheus.
With special affects advanced as they are as evidenced in the scenes depicting the assorted plagues, it was a disappointment that there was not a scene depicting the encounter where Aaron's rod consumed the rods of the Egyptian magicians that turned into serpents.
But I guess it was more important to focus on extra-Biblical details like raids on Hittite encampments and characterizing Moses as some kind of guerrilla in the tradition of Che Guevara or Emilio Aguinaldo.