The fact that someone speaking for the show can call Lady Stoneheart “a zombie who kills people” is another clear demonstration of the show’s complete lack of understanding of nuance when it comes to characterization. In the books, almost every character is emotionally and morally complicated. They’re more than they first appear. But in the show, everyone is reduced to the the most straightforward tropes. Tyrion is the surprising hero of our tale. Brienne is the tough warrior woman. Catelyn was the nagging mother, and as supportive of war as anyone else, while Robb was the bold young hero. Talisa was the Not Like Other Girls love interest. And on, and on, and on. Things are exactly as they appear on the surface.
In this context, it’s unsurprising that Lady Stoneheart seems unnecessary. The Freys haven’t been mentioned this season, and all of the Red Wedding drama is kind of old news. Plus they already have their “seemingly mindless killer zombies” in the form of the White Walkers. They don’t need two sets of the undead.
And perhaps, in the context of the show, Alex Graves is right. Perhaps Lady Stoneheart would just be “a zombie who kills people.” The tragic contrast between the living Catelyn Stark and the reanimated Lady Stoneheart can’t exist on the show, because the show’s version of Catelyn was already keen on vengeance. And as the show didn’t really treat Catelyn as a protagonist by the end, but as a smaller part of Robb’s story, her return is still shocking but less emotionally impactful. We were never taught to care much about this woman when she was alive. Why then should we be expected to care about the changes she’s undergone now she’s dead?