Nitpick: humans are animals.
More to the point, your distinction (humans cannot be freely killed) is a fairly modern one. Until very recently in human history, it was okay to kill humans too, as long as they belong to some outgroup. Strangers outside the tribe were fair game to kill, rob, or enslave.
Gradually our definition of “who is protected, who is ‘like me,’ morally” has expanded to cover others outside our family, village, tribe, nation-state, and so on. But there have been times when people you and I now consider human were legally classed as sub-human, as animals.
Nowadays all humans are in the protected class (except for ragheads and nonwhite terrorists, of course) and the definition shows signs of potentially expanding further…to chimps, gorillas, apes? Dolphins? Whales? Dogs already have some legal protections. The newspaper this week has featured the following two stories:
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All mentally-disabled children deserve respect and rights. Eunice Kennedy Shriver is a heroine for her work pushing the status quo into realizing this.
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Dogs are about as smart as a 2.5-year-old-human child – and not a mentally-disabled one, either.
Okay, so: dogs are popular, they’re part of our families, they’re smarter than some animals we do grant rights and respect to, they work for (earn) what we give them, they’re a partially protected class – do they have rights?
No, dogs have fewer rights and lower legal status than a black man had in the United States 150 years ago.
But look who’s president today.
Things change. Historically there has been a trend – a fitful and often-reversed trend, a ragged and slow trend, true; but there has been a trend toward expanding the circle of who we consider our kin and to whom we will grant legal recognition.
Has that trend ended here, with the latest arbitrary* definition of who is human and who is not?
Don’t bet on it.
*We share 99-point-something-percent of our genes with chimpanzees; it has been argued with some merit that we are, in fact, one of three species of chimpanzee on this planet. Yet the other chimps are still on the outside of that mysterious kinship line. That’s because they are uglier, dumber, more primitive, less well-spoken, better suited to labor than we are, not human, need our protection because they can’t fend for themselves, wouldn’t vote intelligently, like being subordinate to us, are inferior, are not God’s children, have no souls, are property, or whatever. Use any of the these excuses recycled from the African slave trade that appeal to you. Just be aware that they don’t seem to work permanently.