I myself belive that humans are omnivores, with a serious tilt towards the vegetarian side.  But I do think that humans are designed to eat a certain amount of meat in conjunction with a large amount of veggies.  For example, the hunters of hunter-gatherer societies would often exert much effort to chase down what little meat was around (let's say a rabbit).

The difference between now and then was that what small amount of meat was on the rabbit would be shared among the family of the hunter, so everyone got only a little and just enough to satisfy daily protein requirements.  So meat may have it's uses (including using cholesterol as sheathing for our nervous system).

The problem is industrialized meat and, as is your thesis, the prevalance of meat in many human societies.  Thousands of pigs, cows, etc. in one area is very detrimental to the environment, and the energy required to fatten them is staggering.  I'm not a vegetarian, but I am much more heavily into the veggie side than the meat side because of the damage these activities have done to our planet.