In "nature", animals rape and cannibalize one another-
ownership of slaves and denying women suffrage was likewise called "natural".
If this is your ethical paradigm, as opposed to one that accounts for the interests of all sentient beings,
you're in good company, historically, with other bigots.
"Free range" is a label with very little meaning legally, designed to justify charging the marginally-guilty exorbitantly more money for the same product.
Of course, some ethical vegetarians and all vegans don't much care about welfarism-
they believe it's wrong to slaughter sentient beings for human consumption AT ALL.
"The harm" in murdering sentient creatures is identical regardless of species.
What do your philosophical musings about your childhood memories of KILLING have to do with the experiential capacities of animals, exactly?
Edit: There also wasn't one "ancient civilization" without slaves- is that really the best ethical argument you can think of? "People dun it for a long time, yo" ?