The emotional reactions of intense sadness, empathy and feeling like some great injustice is being done at the sight or thought of an animal being hunted or slaughtered for food; is a mental disease / psychological disorder. From this stems vegetarianism.
Veganism is even worse, feeling the same things also at the sight or thought of a cow being milked, a bird's egg being taken, or even at extracting honey from a bee hive.
I would postulate however, that if you have one of these mental diseases, it's best if you abstain from consuming said foods until you're able to solve the issue. So it's not vegetarianism or veganism that is the mental disease, but the emotional reactions that lead you to believe that it's wrong to use animals for food.
