Funny

Why do you need a counter argument? X being valid/true has no impact on whether Y is valid/true. Attack an argument on the merits of the argument, not on the lack of merits of an alternative.

That said, the main argument in favor of eating meat is that humans evolved to eat meat, so our bodies need nutrients that are easier to find in meat (e.g. certain types of protein). However, meat was a much smaller portion of our diets in the past than it is today, so this argument is actually in favor of eating less meat, but still including meat in your diet.

The concepts of veganism aren't really at odds with meat consumption. In many (most?) cases, vegans care most about the ethical treatment of animals (as opposed to vegetarians, who are more often motivated by nutrition), and our current meat processing industry is a lot less ethical than it was hundreds or thousands of years ago when most meat was either free range or wild. So I think it's totally reasonable to take a middle ground and defend meat consumption on nutritional grounds while also defending veganism on ethical grounds.

Eh, the real counter argument isn't about their beliefs. That's fine. Most of it is sound logic.

The problem is their insistence on not only being right, but being better.

The part that makes it silly is the assumptions that chain from there being a right and wrong about what we do with dead animals. It's a corpse. What matters is how we treat the living animals, and they are utterly convinced that not only is their way the one true way, but that anyone who believes otherwise is a bad person. I've been using this troll for something like a decade, and it never, ever fails to draw someone throwing around terms like evil, heartless, cruel, psychopath, etc.

That's the thing to counter argue, not any of the ecological stuff, or the need to treat living things well.

That assumption of moral authority is the point of the troll.

Humans are omnivores, and have been for the lifespan of our species

There are a number of important nutrients that humans get from animal products that are difficult to get from plant-based sources, including vitamin B12, which is not present in land-based plant species (I'm not sure whether red algae counts as a plant, so I'm playing it safe with land-based)