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?