John K.
The answer to your question is simple. Quite frankly, you are forgetting that we are, by definition, part of nature and the natural food chain. Would you ask the same question of other animals that are omnivores? Why not ask the shark why it eats fish, or wild cats why they hunt other mammals? We exist within the natural order, and eating meat does not disrupt that order, in and of itself. Along the same line of thinking, eating plants would also impact nature, so how is that any better? And just how would we fertilize all those plants without ‘animal waste’? Think about it.
That being said, in general, humans have a very poor record in terms of maintaining a balance. In our strive to sit atop the food chain, we have overcompensated, and threaten ourselves and other species in the process. As nature always does, it will require that balance to be reattained . . . and we will pay that price, sooner or later. But so have countless other species in the history of the world, when they overpopulate and ‘spoil’ their environment . . . it is the natural order of things. We reap what we sow.