The recent Pet Sematary remake took plenty of creative liberties with Stephen King's novel, and killing Ellie instead of Gage was easily the most unexpected turn. That said, as refreshing as it might be for adaptations to subvert expectations, this change wasn't quite as successful as the intention was inspired.
In King's original novel, once Louis and Rachel Creed move their family to Ludlow, Maine, their lives are as good as cursed. Aside from some bad omens right out of the gate - Louis fails to save the victim of a car accident and the Creeds' pet cat is run over and killed - Louis learns about a supernatural burial ground deep in the woods that can bring the dead back to life. This knowledge sets the course for the family's eventual downfall. When their two-year-old son Gage is killed, Louis inevitably buries him in the cursed cemetery, only to find that the toddler who returns from the grave is possessed by something evil, leading to a devastating final act. However, in the recent 2019 adaptation, Gage is replaced with his older sister Ellie (played by Jeté Laurence), a change that not only deviates from the source material, but changes the tone and emotional weight of the entire film - and not for the better.
Even though it was an interesting decision to focus on Ellie, introducing this new element of awareness to the character, it struggles to live up to the original death. Aside from the subjective visual horror, Ellie's age worked against her. The idea of setting her up as a character who struggled to understand death, only to come back to life after she died was effective; however, the film struggles to maintain the juggling act of making her a sympathetic character, while also depicting her as a threat. One major criticism aimed at most remakes in the horror genre is the humanization of its central villains, and this is one of the key problems with Ellie. She knows too much, thinks too much, and says too much. It opens the door for exposition, whereas the "show, don't tell" approach from Gage's death kept things simpler and far more sinister.
The death of any child is haunting enough, but Gage's death brought a more devastating kind of terror. It's more tragic that he never had a chance; that he never understood the consequences of running into the middle of the road. So, when he makes this jarring transition into a savage killer, it takes a toll. When he comes back from the dead, he still looks like an innocent child, so, not only does he seem harmless, the contrast between good and evil is blurred.
The recent Pet Sematary adaptation makes a strong case for changing the formula in one of Stephen King's most horrifying stories, and on its own, it works. In a story constructed around death, diving deeper into the existential side of horror makes sense. However, compared to the simplicity of the original, Ellie's death doesn't feel nearly as potent. Less is usually more in horror, and in a story where the unhinged and unpredictable response to the loss of a child speaks for itself, substituting Ellie for Gage might have worked better on paper than it does on screen.