The claim here is that it's harmful to feed your pet a vegetarian diet. If you don't want to kill animals, don't get a carnivorous pet.
There's no "magic" nutrient that animal products have one can't get from fortified plants. Sure, a cat needs synthetic taurine. What is wrong with fortifying their foods? Pet food companies do that anyway, even with non-vegan food. So why not investigate vegan cat and dog food companies? A well-balanced diet is a nutritionally adequate diet, no matter the origin of the materials used to make it.
Some animals are recommended a (well-balanced, well-planned, etc., etc.) vegan diet even though they are part of a species that we would think of as "carnivorous". It's even credited as the reason the record-breaking dog Bramble made it to a quarter-century on a plant-based diet. So the idea that plant-based foods will automatically kill a dog is just not true. A diet that is free of animal products can actually provide some provable benefits or be superior. So a vegan diet is unlikely to be worse for your "carnivore" pet; you would need to demonstrate that to make the claim that feeding them this diet constitutes animal abuse.
It almost goes without mentioning, but if you are reading this, then you are not an obligate carnivore in any sense of the term. So arguments about pets don't apply to you. It's still unethical for you, a human, to unnecessarily cause harm because it's purely for your entertainment, even if a cat needs a more specific set of nutrients.