Have you ever watched nature, does a bird ask the worm how it's feeling, does it want to be eaten, or seen a cat toy with a mouse for ages before finally killing it, seen orcas playing catch with a seal, hyenas tear apart a gazelle etc, etc, it is the way of the world, the human spiritual construct of consuming the sadness and pain of animals is counterbalanced by primitive beliefs that suggest eating an animal imbues you with the spiritual vitality of that animal.
What makes the Vegan ideal any more valid than primitive peoples who were much more in tune with their environment and the earth which provides them with life?
We are physiologically adapted to hunting and eating animals, that's just the way it is, if you choose not to then that's fine, justify it as your spiritual direction, why is there a need to prove it with science? Surely the angst involved in trying to constantly come up with shaky arguments to justify your way of life is not good for your emotional health, one can find peace within themselves more easily if you can accept that others may have a different belief stream.
You don't see the Paleo community criticising Budhist monks for being Vegan or Vegetarian because for them it is clearly a fundamental spiritual belief and science is irrelevant, the only reason there is argument between the two communities is that Vegans more often than not try to use selective science to further their cause. Although sometimes even in the Paleo community I have seen argument on who is more right, healthy debate sometimes, can become dogmatic as well. I found it quite amusing to read through a discussion of raw foodies using shaky science to criticise fruitarians for using even shakier science.
I did read through quite a bit on many diet types all the way from Fruitarians through to Inuit style and after all this decided that a mixed Paleo style diet allowed the greatest margin of error to maintain good health and haven't seen any good evidence yet to convince me otherwise.