Vegetarians deserve our respect. They're usually thoughtful, conscientious people who make sacrifices for environmental and ethical reasons. I was vegetarian for a while myself, and I have no regrets about it.
Vegetarianism and especially veganism can get pretty ideological sometimes. People who have strong beliefs like to think that their belief system is best for all aspects of their lives and the world, not just some aspects of it. Many vegetarians believe their way of eating is healthier than omnivory. One of the classic arguments for vegetarianism goes something like this: our closest living relatives, chimpanzees and bonobos, are mostly vegetarian, therefore that's the diet to which we're adapted as well. Here's the problem with that argument:
Where are chimps (Pan troglodytes) on this chart? They aren't on it, for two related reasons: they aren't in the genus Homo, and they diverged from us 5-7 million years ago. Homo erectus diverged from our lineage about 1.5 million years ago. I don't know if you've ever seen a Homo erectus skull, but 1.5 million years is clearly enough time to do some evolving. Homo erectus ate animals as a significant portion of its diet.
If you look at the chart above, Homo rhodesiensis (often considered a variant of Homo heidelbergensis) is our closest ancestor, and our point of divergence with neanderthals (Homo neanderthalensis). Some archaeologists believe H. heidelbergensis was the same species as modern Homo sapiens. I haven't been able to find any direct evidence of the diet of H. heidelbergensis from bone isotope ratios, but the indirect evidence indicates that they were capable hunters who probably got a substantial proportion of their calories from meat. In Europe, they hunted now-extinct megafauna such as wooly rhinos. These things make modern cows look like chicken nuggets.
H. heidelbergensis was a skilled hunter and very athletic. They were top predators in their ecosystems, judged by the fact that they took their time with carcasses, butchering them thoroughly and extracting marrow from bones. No predator or scavenger was capable of driving them away from a kill.
Our closest recent relative was Homo neanderthalensis, the neanderthal. They died out around 30,000 years ago. There have been several good studies on the isotope ratios of neanderthal bones, all indicating that neanderthals obtained most of their protein from meat. They relied both on land and marine animals, depending on what was available. Needless to say, neanderthals are much more closely related to humans than chimpanzees, having diverged from us less than 500,000 years ago. That's less than one-tenth the time between humans and chimpanzees.
I don't think this means humans are built to be carnivores, particularly since there is accumulating evidence of diverse plant consumption by neanderthals, but it certainly blows away the argument that we're built to be vegetarians. Historical human hunter-gatherers had very diverse diets, but on average were meat-heavy omnivores.