Bad Arguments for veganism
Bad argument for veganism are harmful in several ways, partially by displacing good Argument for veganism, and partially by directly making vegans look irrational or illogical, which negatively influences intelligent non-vegans to reject or even oppose veganism.
NameTheTrait
Gross-out arguments
Humans are Herbivores
Appeal to nature fallacy. Whether or not we were herbivores (we we herbivores and omnivores, and even mostly carnivorous at various points in our evolution depending on how far you go back) is not relevant to what we should do in terms of ethics. The relevant question is the harm vs. benefit of practices.
Thou Shalt Not Kill
And other simple misinterpretations of scripture.
Scriptural arguments are poor for anybody not part of the religion for which scripture is being used, but even if they are, misinterprting scripture is a short sighted one; all it takes is a simple Google search or a two minute conversation with a pastor or educated believer to reveal the mistake.
"Thou shalt not kill" for example, is better translated as "thou shalt not murder"; in many instances killing is commanded in the Bible, including of humans, as punishment for crime, in war, and in self defense, and different words are used for these. An engagement with this question is a nuanced discussion of blood guilt and the nature of sin against God. There are strong arguments from Jewish, Christian, and Islamic metaphysics (as well as other religions), but they're significantly harder to make than a one-line checkmate.