Plant food contains (I think) 7 fewer amino acids than animal food. Those missing amino acids used in the generation of saliva, which is the first and the most important defense mechanism in the way to prevent infection. So Vegans need to get these amino acids as supplements generated chemically.

It is an unpleasant thought to kill anything, but it depends on the availability of food. There were many known cases when during the extensive famines people supplicated to eating animals regardless of the brain complexity considerations, even to cannibalism.

Thus, the more abandoned feed sources are produced on the planet, the further people will step away from the animal murder. However, if you go to Masai tribe that lives and dies by their cattle, it would be hard to convince them that killing the cow is bad. First, we have to provide for them an alternative source of food.