vegan because one believes to be healthier: debatable, but rational. The science may be hard or manipulated, but it is an opinion based on something.

vegan because one believes killing animals is wrong: not just wrong antropology, irrational. How many animals agriculture kills? Habitat change to clear land and mess with rivers, mass pesticides, farmers killing even herbivorous wild animals, big machines crunching animals (not only during harvest), food industry pollution, then eternal seasonal habitat change in plantations.

It is just an self-illusion of living without increasing entropy.

On the same level of "milk come from the card box, not the cow"