Darwin’s "survival of the fittest" theory is certainly an accurate description of "nature’s law," which requires carnivorous animals to eat other animals in order to survive. The animals who kill other animals for food do so because they have no choice in the matter - it is a matter of necessity and survival and they would starve to death otherwise.
Millions of vegetarians the world over are proof that we won't drop dead if we stop eating meat, and, whether or not you believe that a vegetarian diet has certain health benefits, in fact, millions of vegetarians who have never eaten meat in their lives (such as those born and raised vegetarian) are proof that one can indeed not only survive but thrive without ever eating meat at all! Thus, eating meat is, truly, a choice, and we do not have to kill other animals in order to survive, and we will neither starve to death nor be negatively affected by not eating animals.
Humans have a social order in our interactions with each other and the animals we love and protect—animals like dogs and cats. Few people reading this article would seriously argue that we should be killing dogs and cats for food.
A vegetarian diet extends this compassion (whether innate or cultural) that most people in developed nations have for domestic animals and the respect of the right to life that we give other humans, to all animals.
After learning about the cruelties that go on in the average slaughterhouse, it would be very hard to argue that what you see is morally defensible, no matter what happens among other animals in nature and in the wild.