This is a sensitive topic with potential for some hurt feelings and offense on both sides of the fence. I personally have no problems with someone who is a vegan or vegetarian for whatever reasons that they may have. If you don't want to eat animals because you feel it is unethical or unhealthy, by all means follow your conscience. BUT...(and this is a BIG but...no pun intended), I have a serious problem when someone wants to force me to adopt their code of ethics or morals and will use emotional blackmail, outright lies, threats and violence to achieve that (PCRM, PETA and ALF's tactics).
Most (not all) of the arguments that I have run into in favor of not eating animals are based more on emotion than they are on sound science.
I'd say that the world needs both vegetarians and meat eaters. If we all ate nothing but animal protein, there would soon be a serious shortage. The same goes for getting all of our food from plant sources. Some people seem to do better on a diet that is heavier on carbs, while others thrive on a diet that is very low in carbs...we are all unique in that respect and one person should not assume or try to force another to adopt their way of eating because it works for them.
Low carb may be heavy on the animal proteins, but it does not completely exclude plant foods, either, nor does it require that you eat meat (low carb is possible, if not more difficult and perhaps a bit more monotonous, as a lacto-ovo vegetarian). The evidence that we are by nature omnivores (not strictly vegetarian or carnivore but needing both sources of nutrition) is heavily on the side of non-vegans. Because of that, anyone who is following a low carb lifestyle correctly should not have any trouble meeting all of their nutritional needs. The same cannot be said for a vegetarian lifestyle without serious knowledge and planning of meals. Getting sufficient protein and essential fats becomes much more difficult without animal sources or some serious supplementation.