This latest Pet Sematary movie also needs to differentiate itself from the previous one that came out in 1989, and killing Ellie Creed instead of Gage Creed is one of the ways it will do so.
Stephen King hasn’t always approved of changes made to his tales, as evidenced by how he feels about Stanley Kubrick’s adaptation of The Shining, but according to Jason Clarke, he doesn’t have an issue with Pet Sematary killing off the other Creed child.
It goes without saying that when adapting a book into a movie, there are going to be changes to the narrative not only so it can fit within a certain runtime, but also stand out from the source material.