I eat meat because I think it tastes good and as a human I am an omnivore. I do not think that just eating animals is cruel, animals (as humans are) eat other animals, it is a part of nature. I do have a problem with the way animals are treated today but if animals are raised and killed humanely I do not have a problem with it. I think that humans have always eaten meat, and the animals we evolved from probably did, too, at least for awhile back.
As far as evolution is concerned, meat is a superior food source it has more calories than plants. Think about it, grazing animals spend most of their day trying to eat enough to survive, however, carnivorous animals don't spend near as much time finding food. NOTE: I am not saying that this is still true in modern, human society. It has just been true for most of evolution. Vegetarian and vegan diets today are much easier and probably healthier than they would have been say 10,000 years ago.
I hope I have answered your question and good luck with your paper!