Oh man ... flame war alert.
The reality of course, is more nuanced. Rarely do absolutist statements like "Killing animals is wrong," bare fruit in the pragmatic reality of everyday experience.
For one thing, freeing all livestock and allowing them to live as nature intended, would be releasing them into a much more savage existence. An analog to freeing slaves does not exist because people have the capacity to improve their station, while animals in the wild fall back to basic survival.
My wife and I run a backyard hog farm, raising animals on pasture with ready access to clean water and plenty of highly nutritious food. In return for the kindness we show them, at a pre-determined point in their life, we kill them and eat their meat.
I will not do that for my children, because the (mostly) tacit contract I have with them and their own capacity allows them pay back my kindness and generosity with their nuanced human emotion and industrial capacity.
Most animals do not have such an industrial capacity. Though where they do (horses, mules, etc...) there actually does exist a very different contract (i.e. we don't eat them).
The problem with your black and white view of morality is that it fails to account for the reality that if I keep 15 hogs, eventually they will die, because all life dies. If they live into old age and become emaciated and weak, their bodies are useless to me. Aside from perhaps companionship, they have not had a net positive impact on my life. Why then should I care for them?
As members of the Animal Liberation Front have discovered, releasing animals back into the wild is rarely a kindness to them. If you ever look closely at a wild rabbit, you find them swarming with parasites and usually living with chronic low-grade infection of some sort or another. Compare that with rabbits raised humanely and holistically in a small-scale rabbit farm and the differences are stark, indeed.
None of this applies, of course, to an industrial farm where animals are treated, as a phase of the end product instead of animal. But that's a whole different argument from "killing animals is wrong." I have no problem arguing that industrial farming is, at it's best, amoral, and more importantly completely unsustainable. But nature has a way of handling unsustainable behavior.
EDIT: Dairy animals would be another classification of animals that we do not kill for their meat. Of course, they do die, as one of our dairy goats did a few days ago. But then we dig a hole, say some kind words and let the decomposition feed the apple trees in our orchard.