There's more than one type of veggie
Any form of meat production is likely to be grossly inefficient, maybe a bit better than whole cows but still wasteful. Some veggies see that as a motivation in a world where food costs are rising.
Many of course are bozos, persuing an over romanticised ideal that ignore the basic biology that we not only can eat meat but as omnivores we do need to consume at least some.
Then again a friend of mine is an Oxford PhD biologist who just finds cows disgusting.
Eating human flesh does have a moral dimension, depending upon your ethical system of course.
There are diseases like Kuru which have evolved to exploit cannibalism and mad cow disease seems to have come about because recycling of non-food grade meat into animal feed was a short enough cycle that something could survive and evolve to exploit this new "environment".
That means cannibalism creates a form of pollution, helping undesirable organisms evolve, indeed as we recycle more, we can expect stuff like that this to happen more.