The question of what to feed pets must be answered by what best serves their health. Although it would be unethical to cause needless suffering and death in animals used for meat foods, it would also be unethical to cause needless suffering and death by forcing pets to eat an unhealthy food.
A case in point is the thousands of deaths and untold suffering of cats from a deficiency of taurine (an amino acid) in commercial cat foods. These pet foods were deficient because the meats used were processed, which resulted in the loss of taurine.
A vegan diet is essentially devoid of taurine. Its essentiality is one of the reasons pets are naturally carnivores. Is it ethically correct to doom pets to suffering and death by feeding them a diet they would never naturally eat in the wild, and for which they are not genetically adapted?
For these reasons, even though Wysong Vegan™ is a complete pet diet, we cannot recommend that it be fed exclusively.
We are very much empathetic with those who feel ethical anguish about the death of animals used for food. However, life is not possible for one creature without the loss of life by another. A cow kills grass, a cat kills a mouse, a whale eats a fish, an elephant destroys a tree, an immune cell destroys a bacterial invader, and so forth throughout all of nature. This is the truth of the world in which we live. It is real and unavoidable. Life necessitates death.
We may not like the fact that sustenance of life requires the taking of life, but that does not change the fact. We can try to avoid this by creating arbitrary definitions, for example, condoning the killing of non-"sentient" creatures and those without a "brain and nervous system." But, who gets to decide what "brain," "nervous system," or "sentient" is and who gets to be lucky and fall under the rubric that grants life? Who decides how to draw lines when in reality there are no clear demarcations among life forms? True, a blade of grass appears clearly different from a cow, but they are both equally alive. One just appears to be more different from us than another. But in reality there are no clear separations except those we artificially and arbitrarily impose through the use of words and labels.
The more we learn, the more it becomes impossible to unequivocally classify which life forms are deserving of death as food, and which are not. Although plants may not be able to yell or run, they are able to communicate chemically and react to stimuli. They defend themselves with toxins rather than fang and claw, and struggle to survive just like any other creature. Some can live thousands of years.
Our heart tells us to spare lives of creatures we perceive as being most like us. But this is a luxury of the food abundance of modern circumstances. In the wild, creatures eat what they can to survive. In the wild, no pet survives on vegan foods. It would be foolish not to pay attention to what nature teaches.
Also, consider that vegan foods are only possible due to modern agriculture. But this agriculture lays waste to natural habitats that were home to millions of creatures of every sort. In other words, the wheat, corn, soy, veggies and fruit we buy at the grocery store are not only living creatures themselves, but their production has caused the death of millions of mammals, birds, fish, insects, etc.
We can kill and eat some of the deer in the forest, or we can plow the forest under, cause the deer to starve, and kill the plants the farmer grows.
Ethically, the former seems a better choice. But in any case, there is simply no way to eat or feed our pets without causing death.
Given that, our heart must also tell us to properly care for our pets and protect them from disease. Strict vegetarianism (veganism) risks the health and life of pets specifically designed to eat meat. There are nutritional elements in meat products not found in plant materials. You can try to get around it with synthetic additives and the like, but that forces pets to eat synthetic, primarily cooked concoctions they would never find, nor eat in the wild. Such departure from the natural template will surely result in health consequences.
Those who seek a more just and humane world try to listen to their inner voices and treat all of nature with love and respect. Our mind, on the other hand, forces us to face the reality that feeding improperly is a cruelty. Is it any less cruel to make an obligate carnivore such as the cat "go meatless" than to keep a fish, but not in water? The consequence may be delayed for the cat, but is just as sure.
There is no physical or biological certainty as to what is or is not ethical to eat. There is only certainty about what is or is not healthy to eat. The food a creature is genetically adapted to is the healthy food. If we violate this law, cruelty in the form of disease, suffering and death will result. It is, therefore, a choice of whether to, as humanely as possible, take the life of others for the sustenance of nutritional health, or arbitrarily make choices that will cause disease, suffering, and death of ourselves and the pets in our care.