Regarding factory farming vs. life in a state of nature: on average, each breeding pair of animals will produce just two offspring that live to maturity and reproduce. Which means that the vast majority of animals ever born will die of something other than old age. And mostly those deaths will come via predation, starvation, or by freezing to death (in northern climates). In other words, the scale of death and misery in nature is massive.
And regarding my second point somehow justifying human slavery: that's only true if your moral system somehow makes humans and animals out to be equivalent. But we don't even make different classes of humans morally equivalent to each other, so why would we make animals morally equivalent to humans?
A life of pain and suffering on a factory farm is worse than a natural life, where there is at least the possibility of joy and natural behavior. The only thing many factory-farmed animals ever touch is steel, concrete, and their own waste; they never see the sun nor feel the wind. Many are artificially inseminated, so they don't reproduce naturally. Many never see live plants, let alone eat them, so they don't feed naturally. Many live in cages so small they can't turn around or stand up, others so packed together that they go crazy and resort to cannibalism. To mitigate or prevent cannibalism, pigs and chickens are mutilated in various ways. From lack of exercise and intense weight gain many can't stand on their own for the later portions of their short lives. The scale and degree of cruelty that factory farming inflicts on animals is really incredible, and I think it says something horrible about humanity.
And regarding my second point somehow justifying human slavery: that's only true if your moral system somehow makes humans and animals out to be equivalent. But we don't even make different classes of humans morally equivalent to each other, so why would we make animals morally equivalent to humans?